When I was about 12 I read a book which stuck in my memory, but I haven't been able to identify it. It was set in the future when some sort of environmental disaster had occurred and the birthrate had been affected. Aliens were fully integrated into Earth life. The main characters were girl/boy twins (commented on as being incredibly rare) aged about twelve. They visited a teacher (alien) who had recently had a baby - the whole class (or school?) went as it was such an unusual occurrence. London was frozen and the climactic scenes take place in an ice bound and abandoned St Pauls, but I can't remember why! The book cover had frozen St Pauls in the centre. I bought it from Booksale in the early 1990s.
I don't remember it being that good, it just irritates me that I can't remember it!