There's a jurisdictional difference between requirements for "equipment" (which is determined by the state of registry) and "radios" (which is determined by the state you're flying through).
For the UK, for IFR in controlled airspace you need:
* COM radio, must be 8.33 capable from 1 Jan 2014
* Mode S transponder
* Radio communication and radio navigation equipment capable of enabling the aircraft to be navigated along the intended route including: (a) ADF (b) DME and (c) VOR.
* RNAV5 system (effectively an IFR approved GPS) is required for enroute controlled airspace (airways).
There is now an exemption that allows you to fly enroute without the ADF, but it is still required for many approaches.
Under EAS OPS rules, the specific requirement for ADF/DME/VOR will disappear. The rest is the same. If you (the operator) are resident in the EU, you will also need to adhere to EASA OPS rules in their entirety, though the equipment requirements are not onerous (the six-pack).