In no particular order.
Both Carphone place and Argos do "offers" on yesterdays must have phones. Drop in and have a look.
For a first smart phone, you do not need a Nexus/Galaxy S2/Iphone whatever. (You do not need the £400 ++ flash phones).
Start with a reasonably cheap job such as a Galaxy Y or Galaxy Ace or a W, and expect that to last of for sometime. My Galaxy Apollo (no longer sold, unless BOAC has found yet another source for them) is now a couple of years old and is fine. It is v. slow compared to daughter's Galaxy S2 which is lightning quick, bit I'm not lightning!!
I always buy sim free, and have sim-only "contracts".
I get more than I use for £15/month.
Last month, Tesco had a better deal, sim only, for £12,50 month.
I use Vodafone as they are sometimes honest. Honesty has not been a notable attribute of my experience with 3.
I turn off the data thingie when I'm not using it. My allowance is half a whatever. I've never got near to my limit.
Daughter does not turn off her data access, and does not hit her limit of half a whatever.
Turning off the bits of the phone that costs loads when you go abroad is very, very easy with Samsung.