The expiry of your SEP class rating has no direct effect on the validity of your FI certificate. You may continue to exercise any of the privileges of your FI certificate that do not require you to act (or be able to act) as PIC of a SEP aeroplane.
There is no requirement for a minimum number of flights in a specific timeframe to retain your instructional privileges during the validity period of the FI certificate. Even revalidation or renewal may be accomplished at the appropriate time, without having completed any instructional flights, by attending a seminar and passing an assessment of competence.
To expand on deltahotel's comment; UK issued EASA licences, ratings and certificates are no longer recognised as valid for use on any aircraft registered in an EU member state.