Thats not a bad price for the Canon 55-250mm. And, yes the pair of lenses will
always give you better quality than a superzoom.
You've got a tricky situation here as for £400 to replace
both lenses you're not going to get much more than you've already got, perhaps just IS.
If you can stretch to it, I would get the
Sigma 17-70mm f/2.8-4.5 which is around £250 and the
Sigma 70-300mm f/4-5.6 OS which is abour £300 so it's £50 over budget but you might be able to find them cheaper.
If you cant' stretch that little bit further then I'd get the Sigma 17-70mm and the Canon 55-250mm IS.