If you're flying under the FAR's, ie. N reg then the safety pilot must be minimum of an FAA PPL holder who is current and qualified on type. This means if you are doing ME IR currency, the other pilot must hold a ME rating (they must also sign your logbook). There is no requirement for them to hold an IR if you will remain in VMC.
in the UK the same applies, so long as you're both qualified and current on type, there is nothing to stop you practicing instruments, you under the hood and your buddy acting as safety pilot in VMC. When I say qualified, I mean if you are in a G reg aircraft, you must both hold an appropriate licence to act as PIC of this aircraft. If you mate happens to hold an IMC or IR then you can enter IMC.
Cheers
EA
