Tubs

I've got a spare tub and roll of tape in a saddle bag (used to be toe-strapped under the saddle rails but kept swinging around)..

For London commuting I'd not bother with a spare. Worst case and one gets a flat, one can always ride it.. Unless its a major blow-out--- then you can view it as trashed---- one can just keep adding some air.. If tubulars are properly cemented they'll hold..
As to spares... With glued tires one does not use mastic in the field.. just replace the tire... the pressure of inflation and the heat of braking a few times is enough to get a pretty good bond..
Also... if you are going to carry a spare... you should inspect it from time to time.. I've seen more than a few spares that were in worse shape from abrasion than the tire they were intended to field replace...