Some of the reports I've read suggest that passenger services have been run at 140mph on the
ECML▸ , but I cannot confirm this for certain. My best evidence for this is the fact there is
a 140mph linespeed board from the ECML in the National Railway Museum and I'm pretty sure they wouldn't put up special linespeed boards for trains with special permission to overspeed, such as when 91110 did over 160mph.
traintesting.com reports that:
the line speed between Peterborough and Grantham was increased to 140 mile/h to allow the 91's to show their paces. This was the famous Stoke Bank where the Gresley Class A4 'Mallard' had broken the world speed record for steam in 1938. The signalling was modified to have a 'fifth aspect' utilising a flashing green to indicate that there were at least two blocks clear ahead of the train. Many was the time we had to brake for the last curve in the Down direction (uphill) which had a 135 mile/h restriction!
I think that's probably the only section of
UK▸ 'conventional' railway ever officially given a linespeed above 125mph although special arangments allow non-public workings above the official linespeed at times, as suggested at the same link as above:
We also undertook a few demonstration runs when we ran at 140 mile/h, where the line speed was 125 mile/h, all the way from KX to Edinburgh arriving there in the record time of three and a half hours!
As ChrisB says ERTMS▸ is required first and with ETCS▸ Level 2 being installed on the GWML▸ over the coming years that will become possible, but with lineside signals not being scheduled to be removed until 2025 that is the earliest that speeds could be increased - hence my ten years comment.
Not sure how it works, but you might not have to remove the lineside signals. If the in-cab system is overlaid with the existing signals and the on-board kit is guaranteed to always agree with the signal aspects...