CG77570 - Gains: disposal of found objects: treasure: rewards
If treasure is retained by the Crown ex gratia rewards may be paid, see CG77528. These have no tax consequences. The rewards are pure gifts of cash and are not chargeable to Capital Gains Tax.
Where treasure trove was retained by the Crown, a cash reward may have been paid to the finder. This too was a pure gift of cash not chargeable to Capital Gains Tax.