By using small plugs and sockets such as BS546 2 amp or 5 amp. Simples!
So let me make sure I understand your plan A correctly.
1. I purchase a 2A or 5A socket at a higher price than I would for a 13A socket.
2. I purchase a 2A or 5A plug at a higher price than a 13A plug.
3. I purchase a length of 3C flex capable of 5 or 6A.
4. I purchase a 13A trailing socket.
5. I spend the time fitting items 2, 3 & 4 together
6. I plug the plug mounted power supply that came with the aerial amplifier/CCTV camera etcetera into the lash up.
Alternatively my (and for that matter pretty much everybody else on here) plan B:
1. I purchase a 13A socket.
2. Delete.
3. Delete.
4. Delete.
5. Delete.
6. I plug the plug mounted power supply that came with the aerial amplifier/CCTV camera etcetera directly into a 13A socket.
Do you know what, now I read your suggestion I think I'll do it your way...
But then I'd be stupid too and be criticised by all the others. Alternatively I could do it the easy way, the sensible way, the cheaper way, in fact the better way.