According to those that know....
10-inch subs won't produce the ground shaking low frequency effects you might like, so no matter how many you have you won't get the same effect as a single 15-incher.
10-inch subs typically only go down as far as 35 to 40 Hz, and you need them to go down to 20 or 25 Hz for the floor-shaking to begin.
I have two Polk PSW110 10-inchers in a 19 foot x 12.5 foot dedicated home theater (sic) room and that's certainly true. The -3dB limit is 38 Hz. I bought them, BTW, as factory seconds, more or less at half price, but I wish I'd just bought one bigger one instead.
Go for a single 12 or 15 incher as your budget dictates but make sure the frequency response and the -3dB cutoff in particular goes really really low. I have no idea, unfortunately, about brands for sale in the UK these days.