What mobile phone should I go for out of these.

HTC Desire S
HTC Desire HD
HTC Incredible S
HTC Sensation
Samsung Galaxy S2

What tariff and network?

I don't want to have a big upfront cost and would prefer an 18 month contact.

I would want to pay not more than £30 p/m

I would only need the following tariff.

100 minutes
300 texts
500MB internet

How good is the screen on the Samsung Galaxy S2 compared to the others?

What are the phone screens like to look at when outside on a sunny day?

Thanks in advance.

Sounds like you've pretty much made up ya mind