Edit: Sorry looks like you need to purchase £10 top up as well.
Could be worth a try.
Just checking my topcashback account and they have a 1 day only deal on T-mobile PAYG handsets of £11 cashback.
Had a look at the T-mobile site and there are 3 phones at £9.99 so £2 profit for a back up phone if it tracks.
£9..99 phones are: