JAR-OPS is does not specify 700 hours. That is an error in LASORs! It is a figure for flying single-crew in multi-crew aircraft certificated for single crew operations. It is also incorporated in many companies' ops manuals, making it mandatory for those operations.
I believe you can operate single-engine under IFR in the UK, just not in IMC or at night. This is such a restriction that single-engine operations are not viable under an AOC.
Portsharbourflyer
Don't get confused between VFR/IFR and VMC/IMC. While most aerial work is indeed in VMC, due to airspace restrictions often has to be flown under IFR, at least in the UK. VFR is not allowed in class A airspace, and special VFR clearance is not given except in Jersey and Heathrow zones.