I've been waiting for a decent deal for the HTC 10 for ages. I think this is pretty good from Mobiles.co.uk and have just had my phone dispatched. 3GB is plenty for me although appreciate it won't be enough for everyone.
£25 a month and £75 up front for the phone. However, use code "ELF15" to knock that down to £60. Bear in mind this phone is still £480 to buy new and this effectively makes the phone £480 and line rental £180 over 24 months or £7.50 a month. Also this is *not* one of those annoying cashback deals where you have to pay more and then apply every 3 months for cashback.
You can also try going through Quidco for another £38.50 cashback, although this might not work with the voucher - so you could opt not to use the voucher to make an extra £23.50 through Quidco.