the main difference in design is that some are wax and some are liquid. The liquid ones react quicker to temp changes, apart from that the main considerations for me personally are looks and likelihood to fail.
For price/quality compromise you can't go far wrong with a danfoss RAS-C2 or D2. For a better look go for the D2 or the honeywells. Pegler Terrier valves are good for a budget, you can get them rebranded as Bulldog in plumbcentre. Not as good but very reliable.
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