I’ve used a companion voucher to book seats from Doha to London in March 2026. The voucher is valid to June 2026. Am I right in thinking that I can use the remaining ‘half’ of the voucher to book a flight from London to somewhere, but must book it before we fly Doha/London in March and for a flight that happens before the voucher validity runs out?

What restrictions are there on how far I can fly from London? Am I limited to Doha or just limited to the same mileage? Many thanks!