I have posted on your other thread... Give Southend ATC a call. They are non-Nats but are all trained ATCO's, and operate to the same standards.
Obviously try to get to the larger NATS units, but as its on your doorstep, it seems an obvious port of call.
There is also a bit more diversity with the type of traffic. I.e Boeings mixed with Cessna's and trainers. It all helps.
As I said before, they are rather partial to biscuits!!!!
I just went to flight operations and asked. They weren't busy, so was allowed up. A call first is prefered though. Sorry no number to hand.
Good luck!