I would have thought a normal 3P switchfuse would do - e.g. https://www.tlc-direct.co.uk/Main_Index/Distribution_and_Switchgear_Index/CMD_Switchfuse/index.html - easy enough to gland into too.

As for going into the meter box, I suspect the official answer will be no - so the usual answer to that if there's nowhere convenient indoors adjacent to the meter box, is to install a second enclosure next to the first (often a normal meter box is used).

   - Andy.

Thanks Andy was thinking the same but due to limited wall space looks like I am going to have to stack 2 meter boxes vertically. 

If it helps there are slimline versions available - e.g. https://www.bes.co.uk/flush-fitted-electric-meter-box-19287 (provided your chosen switchfuse will fit of course).

    - Andy.

thanks that should work.

I think that you would struggle to find a TP(S)N switch-fuse which would be small enough to fit in a single meter cabinet with the usual tackle.

FWIW, we have a switch-fuse in a separate box next to the meter box. No reason why they should not be one above the other except that in our case there is a window above. The SWA runs under the adjacent room and then up the wall beyond, so a little less than 12 m. For some reason, 5 core cable has been used. Installed circa 1982, but not for much longer.

The tails between the boxes are enclosed by a short length of conduit.