I've recently just done it.
Ask yourself these questions.
Can you do the following?
Initial call on the ground and reply.
Initial call in the air and the reply.
Position Reports.
Direction Finding calls.
Route Clearance.
Conditional Clearance.
Mayday call.
Pan call.
If you can do the following then take your comms written and take the practical. Theres no tricks involved with the RT Practical. Look at CAP413 on the CAA website too, that tells you what to do in each instance.
I think unless your RT in the air is very poor then 150 pounds for a 3 day course is a waste.