Im riding in london and its cold right now
Pretty sure its your jacket.Im using a merino base then wicking nike shirt and normal cycling shirt and on top my gortex jacket which really breaths well.If your top layeris trapping the heat you will sweat,simple
Ditto - I'm in London, and I reckon its cold
Also I just use a long sleeve merino base and a rapha softshell jacket on the top, long bibs, decent gloves, merino hat and toasty feet (a few options dependant upon how cold and how wet). If I start to overheat, which isnt likely, a half zip on the jacket soon cools me down. Thats warm enough for riding, but I do get cold if I stop.
That said, this is for longer rides, if I were doing an hour of intervals or something then I'd replace the softshell with a long sleeve top.
As mentioned previously, just sounds like the OP is wearing too many layers - take one off and see how you get on!