TB10 Not a bad aircraft......... however!
The TB10 is not a bad aircraft but the shortcomings that have been outlined above show that it will cost more than your C172 and won't work as well from sticky grass airfields in the British winter.
I would think that the DR400 would be the best aircraft to replace the C172 as it WILL lift as much as the C172 off a SHORTER grass strip.
From what you have said you need to have at least the DR400-160, this is the most efficient of the DR400 range in terms of speed/range and unless you are into long flights with a big payload.
The DR400-180 is not a lot faster than the 160 but it can with the supplimentry tank option stay in the air for almost seven hours, this makes the sothern Spain a one stop trip from the UK.