I agree with Steven. Friends First offered this minimum guarantee at a time when it was a lot easier to generate a return of 6%+ per year, but inflation was higher too. They're now contractually obliged to continue to provide a minimum of 4% growth. In order to provide that guarantee, the actual investment content of the fund is very conservative - versions of the Friends First With-Profit fund have over 90% in Bonds at present.
So my speculation would be that the 4% annual bonus currently exceeds the actual returns the underlying assets in the fund are producing and therefore you're unlikely to get a better return any time soon, nor are you likely to escape the MVA any time soon.