First off welcome to the forum. I think you have a sound strategy regarding sovereigns as your fractional gold as they have a low premium.
Personally, if I were in the UK I'd stick to silver Brittanias. So that both your gold and silver stack are capital gains tax exempt.
As you are just starting out I'd steer clear from higher premium silver like the Landmarks of Brittain. Wait a while before buying things like that, unless it's something you just want and don't care about any financial loss on it.
Buying silver from Europe tax free is advisable. You can store your purchases for up to six months and get it shipped in one go to save on shipping.
Buying sovereigns in the UK is easy and tax free. Check out some of the bigger dealers like Chards and Atkinsons.
For best prices check out the gold price checker made by one of the forum members.
If you spend some time reading up on the forum you'll find some great information and experience in buying by other members. There is a list of dealers and also a 'buy and sell' page where you can pick up some great deals.
Good luck with your stacking.