Are any of these valve tuners? If so the sensitivity will be very poor by transistor standards. In any case, clear stereo from Wrotham should be possible, it's the same distance as Sutton Coldfield is for me. Don't go looking for bigger aerials, there are none on the commercial market now and in any case you'd only get another dB or so. Until people stopped buying rooftop FM aerials there were still on the market the original design produced in the 50s when the band was 88-98MHz. At about 101MHz the gain was zero and above that the direction was reversed. If yours is a Triax you'll be fine with that.
There are plenty of general purpose pre-amps available covering Bands I to V. Choose one with plenty of outputs for all your tuners, and of medium gain. I'd go for a masthead type, even if you only use it indoors. There used to be bandpass types for Band II but valve front-ends won't be bothered by strong out-of-band signals.
Take a look at, say
Grax UK