If you’re eligible to upgrade your PAYG phone via Carphone Warehouse (check out the terms here - scroll to the bottom for Ts and Cs), they’ll sell you the Windows Phone 8-rockin’ HTC 8S for £119.95 on O2, T-Mobile or Orange.
Carphone Warehouse has an online checker for eligibility. Just click 'View Deal' then 'Perform Check'.
No top-up required.
(Thanks to aramaj36 at HUKD)