I have mentioned about heavy/light cats but have realised I did not actually give any sort of reference. I reckon our cat is a light weight/performance cruising cat, and she is 36' and is about 4,300kgs loaded. I say "about" as it depends on how much stuff we load on board. Some might say she is a bit heavy, but she sails well enough for me.
So at a rough guess, if you are looking at a 40' cat and they say she weighs about 6,000kg fully loaded, she should perform ok. If she weighs 6,000kg unloaded I would say she was too heavy.
Another indicator is at what wind speed does the motor go off and the sails go up. If it will sail in 5kts as good as it motors, it is good in my eyes. If it needs 15kts before it will sail, well then it would not be for me.
I think you should be able to get a good boat for the money you are looking at, and should not have to spend a further 25k pounds.
Anyway good luck and enjoy the search.