Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,186
Reward flights on Vueling are NON-REFUNDABLE. The offer to move to a different date is a fair one - I'd take it.
You are also completely allowed to fly to Spain (and back to the UK). Whether you are able to (or choose to) self-isolate upon your return to the UK is another matter...
Your only way out is if Vueling completely cancels the flight, in which case you are entitled to a refund.