I think your Android choice boils down to cost firstly, £200 for a small starter Android, up to £400+ for the speedy large screen Androids. This cost is absorbed by your contract monthly fees though so shouldn't be too painful to pay for.
Next is screen size, 3.2 inch screens on the smaller Androids (which is fine but a bit small for web browsing especially if your eyesight isn't what it used to be), ranging up to 4.3 inches on phones like the Desire HD (which is great for browsing, but could be considered too large for some hands and pockets).
Assuming then that you can go for any phone in this price range, go to a shop and hold each model in your hand or find a friend who has one. Slip it in your back pocket or your bag to give you an idea of how it fits in your hand.
As Bagsbunny mentioned, the HTC Wildfire S is a good phone in comparison with the Liquid Mini, and HTC have a good reputation with Android phones so far, Acers are quite new to the market and may be less proven.
I went for a Desire last year, and talked my wife into getting a Desire S this year, both have the same 3.7 inch screen which is a compromise I felt between a decent reading size screen, and having a bulkier phone. We are both very happy with Android phones, and I wish you luck in your choice, you can't go far wrong!