They used a Strategic Airlines A320 in MAN this summer past as a back-up aircraft over the peak period. So could be doing the same next year aswell.
There is to be a B757 at MAN and a B733 at LBA on full-time standby next summer so this could be for additional ad-hoc cover.
Remember G-LSAK is leased to RAK Airways until the end of June. So they will be down an aircraft until then aswell.