Welcome from London.
Get yourself booked on a stalk or two to see if you like it. You'll have to use the estate rifle if going for roe or bigger - muntie and CWD are fine with the .223.
There are some good guys that advertise on here - have a look at the ad section and get one as close to you as possible.
Keep the receipts if you decide to add a .243 (or whatever deer calibre strikes your fancy) as proof you've been stalking.
Think about booking a DSC1 course. It isn't mandatory but it will teach you a fair bit about the 6 UK species.
This is all quite simple.
Then try getting a permission of your own for deer or joining a syndicate- that's the harder part.