This is the Nokia Lumia 625 on a 24 month contract, prices are:
£10 per month (6 months half price) for 300 minutes, unltd texts, 600MB data: Total £210, average £8.75 per month
£12.50 per month (6 months half price) for 500 minutes, unltd texts, 1.5GB: Total £262.50, average £10.94 per month
(The £22.50 per month contract option is poor value.)
The 635 is currently available PAYG for £50 plus it seems £20 top-up here: http://www.hotukdeals.com/deals/nokia-lumia-635-49-99-10-topup-ee-special-edition-dan-phil-sim-only-2067788
However, the 625 is CONSIDERABLY better than the 635.
4.7" screen instead of 4.5".
Capacitive buttons instead of onscreen buttons (so you get more space useable) + separate camera buttons.
Front camera instead of no front camera
LED flash instead of no flash (mainly useful as a torch - this isn't a great camera)
The 625 is generally on sale for around £70, so you are effectively paying £140, or £5.83/month for:
300 minutes/600MB data - This beats the market-leading Tesco £7.50/month sim-only 250 minutes/500MB easily
or £192.50 that is £8.02/month for:
500 minutes/1.5GB data, which easily beats Gifgaff's 500 minutes/1GB (3g-only) data offering at £10/month
Note that Talktalk are currently using Vodafone's network and are 3G only, although this is 4G. They are switching to O2 and will then offer 4G.
Basically compared to a sim-only deal, the phone works out as costing around £20. Which is a great deal for this phone.