The "do everything" tank - Surly LHT

This is looking quite likely actually.

Front mech could be a 28.6mm band on Rival, and swap out the S500 levers for Tektro V levers. XT V brakes and a medium cage rear derailleur with a 11-34t cassette giving a low gear of 34/34 or 27.3 GI up to 50/11 or 124.2 GI with 37c tyres and 172.5mm cranks.

Talk to me about the Microshift bar end shifters.. why those over Shimano/Sram?

Maybe a slight change of plan due to winning a badly listed Sram Force chainset for £45. It's not compact and comes with 53 39 rings.

That gives 131.6 GI max and 29.6 GI minimum if I use an 11-36 cassette instead. I don't think the difference of 2.3GI between 34-34 and 39-36 will be that noticeable! Definitely won't be needing it in London :D