It's nothing as bad as you fear! From LASORS:
A UK National or JAR-FCL Pilot’s Licence will be
re-issued for a further period (5 or 10 year validity as
appropriate to the licence held) subject to meeting the
following:-
1. hold a medical certificate appropriate to the licence
being re-issued valid for the first day of issue of the
new licence.
2. have held a valid Certificate of Test, Certificate of
Experience or Certificate of Revalidation for any
type/class rating that has not expired by more than
5 years*.
*Applicants who are currently flying under the privileges
of a non-UK ICAO licence and are currently flying an
aircraft type endorsed within their UK or JAR-FCL
licence, may have their licence re-issued by holding a
valid medical certificate appropriate to the licence being
re-issued. A QSP in current flying practice on any
military aircraft type (aeroplane for re-issue of
aeroplane licence and helicopter re-issue of helicopter
licence) may have their licence re-issued by holding a
valid medical certificate appropriate to the licence being
re-issue.
Applicants who are not flying an aircraft endorsed
within their UK or JAR-FCL licence may not have their
licence renewed until such time as an appropriate valid
medical certificate and Certificate of revalidation are
obtained. Applicants may either renew an existing
rating within their UK or JAR-FCL licence or may wish
to transfer a type/class rating that he/she is currently
flying under the privileges of their non-UK ICAO
licence. Renewal of appropriate ratings or the transfer
of type/class ratings is specified in Section F.
So, in the worst case you'd need to renew any Class or Type Rating (a Single Engine Piston Class Rating will suffice), renew your JAA Class 1 medical, fill out the form and pay the dosh.
But remember that, if you don't hold an ICAO (or UK military) IR, 7 years after the expiry of the last IR is the maximum before you'll have to take ALL the IR theoretical exams again....