I would say yes. The likes of pen y fan is well paved so I always find, that even in the highest end boots, the biggest problem in wet weather/ winter is slipping on the smooth rock paths. This could be combated maybe by sandpapering up the soles a bit if you're that way inclined, or simply wearing some of those grippy things to go over your boots, they give postman when its snowy and icy.
Brecon as you probably already know is fairly well pathed, and unless you plan on going off track, I would say they would do you well. Just be careful as brecon can also be quite muddy, and very wet, so if there are doubts over their waterproofness, etc. it might be worth investing in some gaiters for bad weather.
If you have had meindl boots before, then you probably already know, they are usually better for people with wider feet.