OK, well for boys there aren't that many schools up to 18.
The obvious choice would be St Helen's College (if you are talking about Hillingdon town versus just Hillingdon Borough), co-ed up to 11.
You can look at the 3 in Northwood: St John's, St Martin's and Northwood Prep. All boys only and up to 13.
There is Reddiford in Pinner, co-ed up to 11. My DS goes here.
In Harrow there is Orley Farm and Quainton Hall, co-ed up to 13.
You could also look at the ones in Gerrards Cross - I'm not sure I even know all the schools that are there but Gayhurst (co-ed up to 13) and Thorpe House (boys up to 16) spring to mind.
York House in Rickmansworth.
The list goes on, and I'm sure others will chime in with more as my knowledge of boys' schools is more limited as I have only 1 DS but 3 DDs.