P.S. With, say, 10 x 20W CFLs on the circuit, even the cheapest models would
have a stab at.

i.e. APC ES350, 350VA/225W. (~60GBP)

With 10 x 100W conventional, you need 1000W.

i.e.

APC Smart-UPS 1500VA/980W (~370GBP)

Will almost do it, if you swap one bulb for a 60W!

Alternatively, you might get away with a power conditioner, which is
basically a UPS without a big battery. You get no downtime protection, but
they might help with the flicker and maintain line voltage during a
brownout. They'll also get you longer bulb life if you have habitual
overvoltage on your line.

LE1200 1200VA (~35GBP)

Christian.