Barbed wire at ground level then stock fencing above. If you may want to move them around as they grow, teach them about electric (wire better than tape in my experience) while they're little and in a small, securely fenced pen. (So they can't escape through the electric and so learn to avoid it.)
They'll need more space as they grow, you can start them in any patch at weaning size. They won't damage the ground or tree roots much until they get bigger, so if you've a bit of overgrown orchard you'd like them to clear, put them there first and move them out when they start to turn over the soil, so before they damage tree roots.
Pigs outdoors in Welsh winters can be fraught. They can end up belly-deep in cold mud, and that's a welfare issue. You may find you need to bring them into a strawed yard / stable for the wettest part of the winter.
Rare breeds better fed on lower-spec feeds than commercial types, and less of it. I use Allen & Page rare breed feeds, more suited to this type of pig. Any agri merchant should stock some Allen & Page products and therefore be able to get you their pig feed if they don't stock it anyway.
Rule of thumb for rare breed pigs is 1lb feed per head per day per month of age, up to 4 months, then stick at that quantity until done.
My experience is that Large Blacks are slower-growing than other rare breeds, but the meat is awesome, so I found it worth the wait. They run to fat easily though, so learn to check their condition score and adapt your feeding so that you can always just feel their ribs if you run your fingers lightly down their flanks.
The butcher knows all about butchering porkers so you don't need to give them a full cutting list if you aren't sure and don't have specific requirements. You will need to tell them things like whether you want the loins as fillets, chops or bacon (it won't be much and the eye won't be huge, but my goodness it will taste fantastic!), belly as streaky bacon, sausage, belly joints or slices, whether you want joints bone-in or rolled, what max size joints / how many from each hind leg (and whether any gammon / ham joints), whether you want all the spare meat made into sausagemeat or some as diced, whether you want all the sausagemeat made into sausages or some left as sausagemeat. You can say things like "1/3 diced, 1/3 minced (of which half as burgers), 1/3 sausagemeat (of which 2/3 as sausages)". If you love sausages and stews more than roast pork, you may want to say something like "one back leg jointed as pork, one as gammon and a ham joint, half of one shoulder as 2 rolled joints, the rest of the shoulder meat for sausages and diced." Or just let the butcher do whatever comes naturally this first time, then you will understand more about the options for next time.
If you want the trotters, head, tongue, cheek, offal, etc, you will need to say so and arrange to collect probably sooner than the joints etc. Any processing like sausages, burgers, curing bacon or hams is usually extra. Curing takes longer so then you have a third collection, usually 2-4 weeks after the joints and sausages.