You can mount your yellow rear indicators on the parcel shelf in you want.
There is nothing to say that they have to be at the rear of the vehicle.
http://www.opsi.gov.uk/si/si1989/Uksi_1 ... .htm#nsch7
The interesting thing about this, from the point of view of parcel shelf mounting, is angles of visibility, and the fact that the outer angle is 80 degrees. This could be difficult to achieve from the parcel shelf, except that the regulation applies to vehicles:
"A motor vehicle first used on or after 1st April 1986 "
I assume, if you can prove the car was used, on British roads before that, you would be okay.
What is also interesting is that there is no maximum or minimum requirement for size of lamp.
Could you get away with a small cluster of amber LED's ?
However, I assume that the regulations are much more lax in this respect for older cars.