Although it had significant sales overseas, ICL's mainframe business was dominated by large contracts from the UK public sector, including Post Office Ltd, the Inland Revenue, the Department for Work and Pensions and the Ministry of Defence.

It also had a strong market share with UK local authorities and (at that time) nationalised utilities including the water, electricity, and gas boards.

Fujitsu (UK) as the hardware and software supplier has been implicated in the British Post Office scandal, which has extended from the 1990s to the 2020s The ICL brand is still used by the former Russian joint-venture of the company, founded in 1991.

On their return home they quickly produced the Ferranti-Packard 6000, developing the machine, compilers and an operating system (before these were common) and putting it on the market by 1963.

A feature of the Executive operating system was its ability to multitask, using dynamic memory allocation enabled with a magnetic drum as an intermediate random access device.

At the time of the original merger, the company inherited extensive engineering and manufacturing facilities in West Gorton, Manchester; Castlereagh in Belfast, Stevenage and Croydon from ICT, and from English Electric in Kidsgrove, Staffordshire and Winsford, Cheshire.

The company also had manufacturing facilities in Park Road Mill, Dukinfield; later replaced by a purpose-built factory at Ashton-under-Lyne.Ashton under-Lyne's team was noted for working on numerous mechanical innovations in the field of computer engineering.

A state of the art printed circuit board plant was built in Plymouth Grove, Manchester in 1979, however financial troubles within the company forced its closure in 1981.

As of 1971[update], the United Kingdom was unusual in Europe for IBM not having more than 50% of the computer market, although an observer stated that the company constrained the size of its British subsidiary to keep ICL alive.

[citation needed] In any event, most of the original EEC board resigned over the interference as they believed that the 1900 series was doomed from the outset, being incompatible with the rest of the marketplace.

It was the first commercial mainframe to exploit optical fibres for central interconnect, and also introduced a multi-CPU (multinode) architecture transparent to the applications.

Early VME/B customers suffered significant performance and reliability problems, and the existence of an alternative product provided a safety net.

Perhaps more significantly, VME/K was the brainchild of Ed Mack, who had been brought in by managing director Geoff Cross as ICL's head of research and development.

ICL's finances deteriorated during the late 1970s, leading to the appointment of a new management team led by Robb Wilmot and Peter Bonfield.

ICL used the term superstructure to refer to the compilers, data management tools, and transaction processing software sitting above the operating system but below the user application – a category now often labelled middleware.

TME came bundled with a hierarchical database management system called RAPID (Record Access Program Independent of Data).

In this context, ICL's commitment to the emerging microcomputer market was questioned by industry commentators who regarded the DRS 10 as a missed opportunity, producing a machine whose £2,250 price was elevated substantially by a £1,000 network card that could have been replaced with a disk controller to deliver a competitive standalone, but network-ready, CP/M system.

[19] 1985 saw another Rair-derived product, the ICL Quattro, a multi-user system employing a single 8086 processor and 8087 floating-point coprocessor, running Concurrent CP/M, and supporting multiple serial terminals.

[21] ICL also supplied British Telecom with various models in its Merlin range of electronic office products, starting in 1983 with the M2226 small business computer, M1100 terminal, and M3300 communicating word processor, all offering connectivity features.

Sales and support offices were in Belfast, Birmingham, Bristol, Cork, Dublin, Glasgow, Stoke-on-Trent, Leeds, Forest Gate, Hartree House Queensway, St Paul's Churchyard, Putney (ICL headquarters) and Manchester.

The Queensway site was above Whiteleys department store, and had been used in the 1950s and 1960s by the LEO arm of food company J. Lyons to run data processing and an early bureau service.

ICL had established a presence in India in its earliest days, through International Computers Indian Manufacture Ltd (ICIM), a partly owned subsidiary.

In later years ICIM, from its offices in Pune, started to establish a presence in the market for offshore software development and eventually outsourcing of the operation of computer services.

[34][28] Data from Horizon was later used to prosecute more than 900 sub-postmasters over unexplained losses,[35] but it was found by the High Court in 2019 that "bugs, errors and defects" within the system could have caused the shortfalls.

The acquisition shifted the geographical balance of ICL's sales away from the UK, and also gave a presence in industry markets such as retail and manufacturing.

In 1989, ICL acquired Regnecentralen of Denmark, a company with a distinguished history and reputation in that country, but which was best known internationally for its front-end communications handling equipment.

At this stage ICL was developing its own large scale integration (LSI) technology for use in the higher-end machines, designed as a successor to the highly successful 2966 processor (known internally as S3).

As part of the 1981 restructuring, Robb Wilmot – an electronics engineer and former head of Texas Instruments' British-based calculator operation – arrived as CEO in May 1981.

[52] Todd resigned and was replaced by Richard Christou, who dismantled the complicated matrix management structure, rationalised the balance sheet and sold some smaller units.

International Computer Logistics, a British company specialising in IT repair and data recovery, secured the intellectual property rights to the ICL brand in 2014.