I assume you mean you have a CPH number for your holding and a flock number for your sheep - if so, you will need a pig herd number, which you will get from Animal Health.
There are loads of threads on here with similar questions to yours and quite a lot of good (IMHO) information on the website proper - click on LIVESTOCK button at the top of the page.
I've never kept Berkshires but they are often recommended for newcomers, because they aren't very big. We've always had Tamworths - they're fab, but quite big and boisterous (but when we were beginners, there wasn't TAS
).
While I'm not saying "don't sell them through the market", you might find a better return selling to friends and family - unless you are planning to start a serious pig breeding / rearing enterprise. But if you are simply keeping a pig for your own freezer and, since you need to have at least two, a second pig, I'd try an sell it privately.
The amount of space you need depends on your land, how many pigs and how often you are keeping them. If you are buying a couple of weaners in spring to fatten until the autumn, then resting the land for 6 months till the next batch - well, we used to do it on two pens of 30ft x 40ft (but they were trashed by October).
Loads of other piggie people on here will advise.