Quite when the Foreign Office woke up and managed to persuade the govt of the nonsense that this theory was, I don't know. I suspect it took the election of Thatcher in 79 to end it all.
The last of the Brush Romanian subcontracted units was delivered in September 1977. The first unit built by BREL Doncaster had entered service 4 months earlier. Given the QC issues experienced with the Romanian units on delivery I suspect the decision not to place any follow-on orders with Brush/Electroputere was made long before 1979.
Mrs. Thatcher visited Romania in September 1971 as Secretary of State for Science and Education in the Heath government, and returned in September 1975 as Leader of the Opposition (followed 2 weeks later by PM Harold Wilson). In April 1981 she hosted the Romanian Prime Minister's visit to London. Successive UK governments from the mid-60s into the 1980s pursued trade and economic relations with Ceaușescu's Romania, so I doubt there was any Government influence to cease the outsourcing of Class 56 construction (it was probably the opposite).
The decision to 'nudge' Brush into outsourcing from Romania in 1974 presumably came as a result of the June 1973 official visit by Edward Heath's Minister of State for Foreign Affairs Julian Amery to Bucharest, which is where the offer of a state visit to the UK was first made.
There was also a deal to build BAC 1-11 aircraft under licence in Romania (for their own use) - the Queen had the unpleasant ask of hosting the Ceausescus for a state visit to smooth the way.
Brush wasn't the first UK company to outsource production to Romania due to lack of capacity - Britten-Norman Aircraft had started outsourcing final assembly of the BN-2 Islander (suppliedi n kit form) to IRMA in 1968 as it's own factory no the Isle Of Wight did not have the capacity to meet customer demand. In 1972, after B-N had been taken over by Fairey, a deal was signed for outsourcing full production of the Islander and it's Rolls Royce turboprop engine by IRMA. By the late 70s Fairey had outsourced all production of the Islander to IRMA (so at least one of the Romanian outsourcing deals was a success).
The Islander deal was arranged as a 'quid pro quo' for Romania ordering BAC 1-11 airliners from the UK in 1968. That sale eventually led to the ill-fated ROMBAC deal in June 1978 (signed during Ceaușescu's state visit) to transfer the production line for the 1-11 from Hurn to Romania (which ultimately was a failure, with Romania defaulting on it's payments to BAC and less than 10 aircraft being completed, mostly from Hurn-supplied subassemblies).
I suspect allowing the absolutely awful Renault 12 derived Dacia branded Romanian built cars to be sold in the UK was also a part of that trade agreement. They were absolutely terribly constructed with multiple faults which required Abbey Hill (the importers) to run them all through a rebuild production line in Yeovil before they could be sold. They were even worse than the contemporary Yugoslavian Zastava / Yugo Fiat based machines.
Typical faults were disconnected / missing brakes, loose windows, warped body panels, missing wheelnuts, disconnected wiring, no engine oil...... It used to amaze us how they ever got them onto the transporters and ships to get them from Romania to the UK. I can't remember if they arrived via Avonmouth or Tilbury but either way the transhipment must have been a nightmare.
The importation of the
Dacia Denem (their licence-produced version of the Renault 12, dating from 1969) for sale in the UK began in 1983 by Andover-based IVI (Commercial) Ltd (trading as the Dacia Car Company Ltd.) with a single shipload of 750 cars. It was expensive compared to other Eastern Bloc imports and the build quality was very poor. No further batches were imported and it took until 1984 for dealers to shift the remaining stock.
The Dacia name returned in 1984 via a new company, Dacia Concessionaires Ltd of Westbury (part of the Abbey Hill vehicle import group). who started importing the Romanian ARO 10 small off-roader as the
Dacia Duster (Auto Romania had no links to Dacia, other than buying engines and gearboxes off them), and also the pickup version of the Denem. Despite similar build quality problems, the Duster was more successful and importation continued until 1993.