If a District Line train would do (bear in mind the section of Circle Line between Tower Hill and South Kensington is shared with the District Line) then there's a
primary school in south London that has a single 'D stock' (in service until 2015) carriage as an outdoor classroom or something. I don't know what they have done with the interior or exterior - it's only partly visible on Street View
here - looks like it might still be in London Underground livery, the current Google satellite view doesn't show it. Might be closer than the Isle of Wight to get to, would imagine that any filming would have to be at a weekend or in school holidays of course.
There's also the converted 'D stock' trains that were used on the Marston Vale (Bedford - Bletchley) rail line until some time last year - I believe they are out of service and in store - someone else may know where.
I also have a faint idea (I can't remember where I might have seen or read about it) that TFL / London Underground have some mock up station and / or train interiors they use for staff training. Again, someone else might have more idea - hiring this for filming might or might not be possible, but might be a lot less expensive than hiring Charing Cross Jubilee Line platforms and / or hiring a real train for filming purposes.