London Info is still the way to do it, unless you are already talking to an approach unit. The trouble with free callers on approach unit freqs. is you tend to be ignored if its busy - certainly true in the SE of England.
Have handy the departure time, the parent unit where the FPL was filed, the requested joining position/level and the estimate for it.
The workload tends to be vertical the first few times so watch out for CAS !!!