Lots of good advice here, and the salutary warnings of not being too quick to jump into hill / primitive sheep!
Much will depend on your attitude to sheep and sheep-keeping. And how you will gather your sheep and bring them in for inspections and treatments. If you have no dog and not many people, sheep that will follow a bucket
even when they think you might be up to something are pretty much essential.
Now most sheep can become bucket-followers, but some breeds have a much higher propensity to value freedom over food, and are prone to breaking away and making a bolt for it if they think they are about to be captured.
As a generalisation, hill / mountain and primitive breeds are more likely to be of the latter type. Downs types and highly commercialised breeds are more likely to let greed silence their inner prey animal.
Whichever breed or type you settle on, make sure you buy socialised animals. Lambs learn from their mothers; how sheep expect to be treated by the humans that care for them will have a big effect on their behaviour around humans
And of course you also need sheep which suit your situation, and, if you want to breed pure, of which you can source tups from time to time.
You may also need to think about what you plan to do with excess lambs. If you can finish everything and sell as meat, then any breed will do. If you need to be able to sell store lambs, for others to finish, then they need to be lambs you can sell locally.
Also, think about what approach to lambing will suit you. Indoor or out? Hands-on on hands-off? Many hill and primitive types are better suited to a later (probably late April where you are) outdoor lambing, and being left to get on with the job. (Check them with binoculars and only approach if there is clearly a problem.) But if you want to lamb earlier, or for other reasons prefer a more hands-on, involved approach, then a less independent breed would suit you better.
You are clearly in Wales as Black Welsh Mountain is your local type, and we can probably assume hilly and wet too? In which case, some of the tamer breeds may not suit the environment - greedy sheep and poor feet often go together, and wet land needs sheep with good feet...
I've no personal experience of BWM (Black Welsh Mountain) sheep, but think it highly likely that, like most hill breeds, they will be independent-minded sheep with a highly-developed prey instinct. Very suited to your conditions but possibly not so easy to manage as a smallholder. So if you did decide to go for them, I would say try to buy from another smallholder, one who can demonstrate that his/her sheep will follow a bucket into a pen - and allow the gate to be shut behind them.