If I'm flying over the top of you I want to see your Mode C response on my TCAS.
Dunno about the vicinity of LGW, but just about everywhere else, it appears to make more work for ATC if there is only Mode A - "XYZ123, popup traffic, slow moving left 10 o'clock - no height information"
They then have to keep an eye on you just in case you're in my way, and I (or my FO) are worriedly looking outside just in case we bang into you at exactly the time when we're really rather busy on the flight deck, when confirming via Mode C that you're well below us is simple and saves an awful lot of worry.
I've always trained my students ALWAYS to squawk mode C. It gives more information. It can help. It can never do any harm. Why not do it?