Hi,
I think the only good things (two) about LHR T3 are the Virgin Atlantic Lounge and the Revivals Lounge
The newspaper shop( which is a about the only place airside )in T3 for water/soft drinks in small bottles to take onboard always has very large queues.
The walkway from security almost always takes you into... the duty free store.
On the other hand never had to wait too long to clear security.
Regards
TBS