I think there are two groups of West London preps (I don't know about the North). Will keep it just to co-ed and boys schools.
Group 1 - Westminster Under, St Pauls, Kings, Latymer Prep, Sussex House. All continue to be highly competitive for places. Perhaps applications are down for some, but not enough that entrance standards have lowered or that they would have problems filling spaces at any entrance point. If considering these, would highly recommend finding a tutor with experience of these schools to evaluate your child and identify any gaps in knowledge that might need filling for the assessments. Plus - visit them first!! You may find they are not the right fit for your child. Don't be swayed by the brand :-)
Group 2 - Eaton House the Manor, Thomas', Hill House, WCCS, Wetherby Prep, Chepstow, Newton, Falkner Boys, Garden House. Some have more space than others, but generally think all will have spaces available. All are excellent.
RE: boys schools going co-ed. Not because there are less boys. But potentially other reasons - girls can improve school results, bigger pool of students to choose from, parents wanting more academic co-ed options etc etc.