You can only revalidate by experience during the second year of rating validity, otherwise you can do a proficiency check at any time. If you pass a proficiency check within the last 3 months of validity then then original expiry date is preserved, otherwise a new expiry date is calculated.
Reference: CAA Standards Document 14, Section 4, Paragraph 4.2. (Details forfeiture of the remaining validity if you complete the proficiency check early).
FCL.740.A(b)(1)(ii) only has provision for revalidation by experience within the 12 months preceding the expiry date of the rating.
https://regulatorylibrary.caa.co.uk/...aeroplanes.htm