If you have instructor ratings on a JAR licence for EASA aircraft (e.g. FI, CRI, SFI, or TRI for an EASA type), the ratings will remain valid subject to the existing renewal/revalidation rules for the ratings. Subsequently, the ratings may be revalidated/renewed in accordance with Part-FCL. Any instructor ratings specific to non-EASA aircraft (e.g. microlight instructor, or TRI for a non-EASA type) will also remain valid, but cannot remain on the licence when it is physically replaced by an EASA licence. You will need to have a UK national licence with the specific non-EASA rating(s) at that time.
If you have a non-JAR UK licence with instructor ratings that include privileges to instruct for JAR licences or ratings, these will still be usable along with the national licence until 2014 or 2015; see the FAQs - “I have a UK ATPL / CPL (pre-JAA), how will the changes affect me?” and “I have a UK PPL (pre-JAA), how will the changes affect me?”
If you choose to fly non-EASA aircraft only in the future, and so do not obtain an EASA licence, your instructor ratings can continue as before, except that Part-FCL rules will apply for revalidation/renewal. If/when you apply for an EASA Part-FCL licence based on your UK licence, the instructor ratings that apply to instruction for JAR/Part-FCL qualifications will be carried over onto your EASA licence, provided that you comply with the experience requirements for the instructor rating that are specified in Part-FCL.
Note 1: There has been some confusion over whether current UK FI ratings will be convertible to equivalent EASA ratings because Part-FCL requires CPL theoretical knowledge examination for the FI, and this was not required for the existing ratings. The position is that CPL theoretical knowledge is required to obtain a new FI on an EASA licence; but it is not required if the FI is granted under the conversion rules to a pilot who already holds a valid FI under national rules who complies with the Part-FCL experience requirements.
Note 2: Under Part-FCL the holder of a PPL with FI rating may be paid to give instruction for the PPL and LAPL. The 2012 amendment of Schedule 7 of the Air Navigation Order allows the holder of a UK PPL(A) with FI to be similarly remunerated for providing instruction.