Given that the 70mph limit was originally brought in as a safety measure (quoting from Wikipedia):
“The introduction of the 70 mph speed limit
In 1966, at the end of the four month trial of a blanket 70 mph (113 km/h) speed limit on previously unrestricted roads and motorways, speed checks on the M6 in Cheshire suggested that although cars were actually being driven about 10 mph (16 km/h) faster, they were still usually travelling at speeds below the new limit. The crash rate was lower on the M6 in Staffordshire (the better weather was noted too) and continued to fall on the M5 in Worcestershire as it had before the new limit was imposed, and there was no change in the crash rate on the M6 in Cheshire or on the M1 in Northamptonshire”
“1973 oil crisis
Due to the 1973 oil crisis, a temporary maximum national speed limit of 50 mph (80 km/h) for all roads, including motorways, was introduced on 8 December 1973. The 70 mph (113 km/h) limit was restored on motorways in March 1974 and on all other roads on 8 May 1974.
As an initiative to reduce energy consumption, the national speed limits for otherwise unrestricted single-carriageway and dual-carriageway roads were temporarily reduced to 50 mph (80 km/h) and 60 mph (97 km/h) respectively (motorway speed limits were left unchanged at 70 mph (113 km/h)) from 14 December 1974. In November 1976 the temporary speed limits were extended at least until the end of May 1977. In April 1977, the government announced that the national speed limits for single-carriageway roads was to be increased to 60 mph (97 km/h) and that the 70 mph (113 km/h) speed limit was to be restored on dual-carriageways on 1 June 1977.”
As vehicle safety standards have improved over the last 45 years (seat belts, radial tyres, ABS, air bags etc.) and cars are capable of being safely driven well in excess of 100mph (160 km/h), why on Earth would we be considering reducing the limit to 55mph (or more likely 56mph (90km/h) to match the LGV limiter speed set by the EU)?
The biggest issue I have observed (and I cover about 30,000miles per annum) is speed differential: when some ditherer in a car changes lane whilst doing less than 70mph and does not increase their speed to match the traffic already in that lane. Hint: if you are not happy at 70mph (or more), stay in the inside lane unless overtaking even slower traffic, and when moving into an overtaking lane (all lanes except lane 1 should only be used for overtaking), match your speed to other traffic, do not expect them to brake down to YOUR unacceptably low speed.
If you want better fuel economy, buy a more economical car. Do not inconvenience others.