How well would you rate all of the mobile networks, on the following.

Tariff cost.
Upfront cost of phone.
Customer Service.
Network Signal both 3G and 2G.
Tariff Value - amount of minutes, texts and data, for the price you pay.

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

Which network would be a good choice to get one of these phones free (if possible) or with the least upfront cost?

I would prefer an 18 month contract.

Thanks in advance.

This site will help you a lot its very hard to go through them all not knowing what you are looking for.

http://www.best-mobile-contracts.co.uk/

I believe network signal is dependent on your area.