I have a siamese and an oriental and they are fantastic animals, they behave as if they're part-cat-part-dog-part-parrot. They are ever so sociable and seem to really need and love the company of other cats. If you visit a (good) breeder you will always see a pile of at least 3 adult cats curled up sleeping together. Mine always sleep together.
We have a male and a female, I really like the balance, but if I had to have two of one sex I'd go for two males rather than two females.
Anyway, If you are going for siamese/orientals I would strongly advise you to get two kittens rather than one. Two siamese/orientals are almost certainly less work than one! (like parrots) they need a lot of human interacion if kept alone they can be destructive and/or miserable if they don't get it.
They are very intelligent cats and impressionable as kittens, a rehomed cat may have real and dramatic problems settling. That said, if the situation the rehomed cats were coming from were very similar situation I would consider it.
I don't know who issues the official pedigree documentation in the UK, but you need to make sure that if you get a pedigree cat (kitten or re-homed) they are registered with them. Kitten farms are terrible places and the cats they produce are badly socialized (if at all) and are likely to have health issues.
A reputable breeder won't allow her cat to have kittens more than twice a year and the best only allow one pregnancy per year. The kittens should be being brought up in the household of the breeder, not in a back bedroom somewhere but actually in the sitting room with the family. Ideally you shouldn't choose a kitten before you can see the individuals character traits 4 - 8 weeks old is fab. Although, they shouldn't leave the mother until 12 - 14 weeks.
waffle, waffle
Good luck!