The short answer is at about 5-6 months, but sometimes earlier (like 4 months and sometimes later.
Here's the longer, more complicated answer:
Cats are "seasonally polyestrus." This means they come into heat multiple times during their mating season (it can seem like they are in heat continuously - they have a very short cycle if not bred). Unfortunately, their mating season is about February to October (but it does vary). Kittens who become old enough to be sexually mature during the non-breeding season will delay their first estrus (heat) until the breeding season starts again. So for instance, a kitten born in June would be 5 months old in November. Since this is usually past the breeding season, that kitten would probably be older than average (closer to 8 months) when she went into heat.
With your kitten, it's tough to say. She will be mature enough right around the typical end of the breeding season. So if I were you, I would assume she could go into heat this season (at about 5-6 months old) to be on the safe side.
In the US (where we are in general much stronger advocates of spaying and neutering than in Europe), some vets will wait until 4 or 5 months to spay or neuter an animal, some will do the surgery younger as long as the kitten/puppy is at least two pounds. I am a fan of the 4 or 5 month spay. I don't know what the UK laws or the BVA standard or care may be regarding this issue, but you may want to ask other vets to see if they do the surgery earlier if you are really worried.
So your options are:
1) Neuter the male, if possible (you didn't say whether they were littermates of if he was older), before your female has a chance to go into heat.
2) See if another vet will do the surgery earlier, for either cat.
3) Once the female reaches 20 weeks of age, keep her completely separated from the male until you can have the surgery done.
4) Do nothing and risk a pregnancy. However, IF she comes into heat at 5 months and gets pregnant, she will only be 30 days along at 6 months, and you can still get her spayed and terminate the pregnancy surgically. At least, you can do this in the US (for a small increase in the surgery cost because of the slightly increased difficulty) - I don't know if there are greater ethical concerns in the UK regarding this issue - it's another good thing to ask the vet.
Of course, if she's one of those rare cats who happens to come into heat at 4 months, this option could be a problem.