Owing to the fact that the UK does not comply with the ICAO Standards set out in Annex 11 at 2.11.3.1 and 2.11.5.1, and appears to not notify Differences from them!
These Standards require Controlled Airspace (CTA & CTR) to be provided for IFR flights, something which Blackpool, Exeter and others in the UK do not have in order to protect IFR arriving/departing/holding traffic.
IIRC there is nothing in the SARPs that precludes IFR flight outside CAS, and I think it happens in many countries. But those standards refer to the establishment of controlled airspace where it is determined that ATC shall be provided to IFR flights....but the UK also allows ATC to be provided outside CAS, and therein lies the problem.