If you look at a reasonably well specified OPV like a Navantia BAM or the Holland class, they're all in excess of £100m - that's for something with a gun at the pointy end, a helipad at the back and some decent sea legs. The Holland has a pretty useful radar for it's class, but they're also £150m.
As soon as you start adding in some sort of local area defence, you're into £200 million for a small ship with no survivability in a shooting war and limited space and weight reserve to upgrade.
For the RN, corvette, bad idea, plain and simple, they're competing for funds with more capable high end escorts which we are badly short of. More OPV's might be nice for the anti drugs/anti piracy stuff but we've got 4 Type 22's tied up alongside that are bought and paid for.
Spend a tiny bit of money stripping out the SeaWolf mount, directors and some other systems, park a 1b on the bow and you'd have a reduced cost option for running Type 22 on for another decade as a large patrol ship. Not going to happen however.