Vodafone* Sim only deal via Mobiles.co.uk
offers a decent allowance. You'll pay £180 over the 12-month contract, but enter the code 10AUTOMSE at the basket stage and you'll be sent a £100 cheque within two months. Factor that in and it's equivalent to £6.67/mth – the cheapest we've seen for this allowance.
Important: Vodafone will automatically add its anti-virus 'Secure Net' to your plan. It's 'free' for the first three months then £1/mth, so you'll need to cancel via its app or online if you don't want it.