The Systemdek IIx900 and virtually any of the suspended subchassis Thorens, Ariston RD80, Heybrook TT2, STD etc. can all be had for around the £250 mark or less and will give significant improvement in fact if you compare these turntables against those costing up to 5 times as much (and more) second hand there is only a shading of difference as the law of diminishing returns kicks in.I agree with what Lurch said. I don't think you'll get a significant improvement with a budget of £250.
Unfortunately you do not list what you are using in your profile and as a result we do not know how many turntables you have had through your hands in your experience.
Having said that if a person with a dual wants to upgrade into moving coils at some time in the future, the only real route is to change turntable. However, if their penchant is for moving magnet cartridges significant improvement can be obtained by upgrading to an Ortofon 2M black or the upper market Audio Technica Moving Magnets.
At the end of the day it is horses for courses, if you do limited listening on vinyl then the cartridge change option may be the best route but if you really intend getting into vinyl then for the budget given the recommendations given provide the possibility of much greater listening enjoyment.