Adding socket for cycle lighting board...

Hi - I want to be able to carry cycles on the prius using a rear cycle rack. This is straightforward, however here in the UK it's a legal requirement to have a number plate/lighting board as the bike will obstruct the rear lights of the vehicle.

I want a socket fitted in the boot, the lighting board can then be connected through - has anyone fitted an lighting board socket to a european (UK) prius - so has seperate indicators/brake lights? I had a look online and saw some bits and pieces, however the sockets behind the rear lights look different to all those I've seen online (there are two grey round plugs, each with just two wires going into them). The brake lights on my Prius are LED type.

I've asked my toyota dealer and they have said they can do it, but it'll cost £334! (about $515) - seems a bit steep.