I believe that JAR-OPS allows an operator to lease a aircraft from ICAO state but it has to be maintanied and approved i.a.w. EASA-145 maintenance standards and generally the requirements and paperwork required make it not very finacially attractive to do such.
I also think it can only be done for up to 6 mnts max.
This is all from vague memory and off course the UK CAA as usual may have their completely own ideas about this.
Looking forward to hear other inputs regarding this?