The key to this is understanding the signal that a "406" beacon puts out.
It has two components:
The 406 MHz signal, which carries a unique ID (so if the database is correct the RCC knows where the beacon came from and can check where it should be) and a data message that can include a GPS position, is transmitted at 5 watts power for 0.5 seconds every 50 seconds. There is a small randomisation on that 50 seconds so that 2 beacons don't transmit on top of each other forever. Most beacons (including the ones being acquired for the British Military under the PELS programme) do have a GPS. Because of the structure of the digital message, there is a limt of about 4 seconds of arc on the position that can be transmitted. This is nothing to do with GPS accuracy - there just isn't enough space in the message (the structure of which is internationally agreed) to transmit digits to get you better accuracy. The accuracy achievable is a box about 120 metres x 70 metres at UK latitudes.
The other signal is a near-continuous swept-tone 121.5 MHz analogue at about 125 mW. This can be picked up by homing equipment in the exact same way as it has been done for many years.
Some beacons (including PELS) also transmit the swept tone on 243 MHz.
To find someone wearing one of these beacons, the primary method is to get the GPS position and go there. If at that point it's dark and foggy, you carry out an old-fashined homing on 121.5 / 243. You can get the GPS position either via the satellite network (the UK terminal is co-located with the Rescue Co-ordination Centre at Kinloss) or by receiving and decoding the 406 signal directly.
Most modern homing systems (eg the Chelton 7 fitted to the RAF Griffin 2s and the Chelton 9 fitted to the CG SAR 92s and 139s) can home on 406, as well as decoding the digital information. However, 406 homing, although very useful for long-range work because of the power of the signal and the fact that there shouldn't be anything except emergency beacons on that frequency, and quite useful for multiple beacon situations because of the short duration and long gaps in transmission, is very little use for establishing an on-top, because you can be 2 miles through the overhead (well one-and-a-bit in a Sea King) before the next pulse that tells you the beacon is behind you.
So, in theory, the fact that existing military SAR aircraft (Griffin 2 excepted) cannot home to or decode the 406 signal, doesn't affect their capability, provided the situation isn't too complicated and everything else in the beacon / satellite package works as advertised.
Sven