The no-application CEPT (61-01) arrangement is suplemented by CEPT 61-02 - which specifically handles the need to have a temporary reciprocal licence for longer than 3 months:
From the Ofcom (UK regulator) page at: https://www.ofcom.org.uk/manage-your-licence/radiocommunication-licences/amateur-radio/amateur-radio-info (under Frequently asked questions / I hold an Overseas Amateur Radio Licence. Can I operate in the UK?)
CEPT Recommendation T/R 61-02
This recommendation puts in place arrangements to allow qualifying licensed amateurs from other participating states to establish and use amateur radio apparatus in the UK for periods in excess of three months, subject to being issued with an individual licence and call sign by us.
Please make sure you allow enough time for processing however as this is similar to the old reciprocal licencing option that existed prior to the UK (and US) signing up to the CEPT 61-01 agreement and hence some work is involved by Ofcom - I would apply 6 months before the required start date if possible.