I appreciate shelters are not always easy to deal with, but please please do not buy a cat off gumtree. The dealers that sell on there are not good people.
We don’t totally know her history but my cat was likely previously owned by one. She was dumped on a shelter when her second litter all died. She’s a runt and tiny and it’s debatable if she should even have been used as a breeding cat, but sidestepping that she was 18 months old when we got her, and it’s just dangerous putting cats through multiple breedings at that age.
And also I will continue to be a killjoy but please also think about what happens if you suddenly no longer have that pet-friendly accommodation. Giving up cats is very hard to do and London is a pretty horrible place to rent at times.
If you can give a cat a safe, secure and long-term home then it’s just the best and I can’t recommend it any more. Snoopy brings so much joy to my life, and I say this as someone who grew up with cats as well.