It would be worth looking into the specs of your speakers before jumping to the conclusion of buying a sub. If you find £200 too much for a Gemini then I am not convinced something cheaper will give you much more bottom end than the speakers you already have. If anything you might find spending the money on a better amp to power your existing speakers may give you the low end you sek.
For example my Mordaunt Short floorstanders go down to 50hz quite comfortably. Marrying something like a BK monolith with this makes sense, but a £150 sub will add little to no value.
PS a second vote for the BK Monolith, even if it is £250 over budget!