Hi Mark, and welcome to YD
The crossover issue is one tha faces me as well. I am currently a PADI Rescue Diver, but had very few buddies (pre YD membership!), so my diving was very limited. I am looking at a local BSAC club to do crossover, and the good news is that these days you can get an equivalent BSAC membership without having to do all of the training again, but if you want to move upwards in the BSAC training structure, then you *do* have to start again at the bottom. Your local club may vary though, so its a case of having a good chat with them before joining, to make sure that they can offer you what you want.
Whichever system you choose though, you will need to do more training before you can go too deep anyway.
Enjoy the forums, and have fun!