Hi guys. regarding a prior debate about HS1 speeds, I have some info.
I spoke to someone at Railfest who works on the 395s at Ashford depot.
The top speed permitted right across the HS1 line is 140mph - including the tunnels. However, "it is not uncommon for drivers to ease-off slightly when rolling through the tunnel between Stratford and St Pancras" as the gentlemen I spoke to said.
I'm popping down to Ebbsfleet on Wednesday evening so I can still try ask a driver directly what they do if anyone would like to know still

I don't know if what I've been told includes the HS line from Ashford to the Channel Tunnel or just St Pan' to Ashford.
HOWEVER.. the 395 does have a pretty big sign inside saying "MAX SPEED - 140MPH" so I'm guessing that's across the whole line.
All the best!