hays wrote:
Can a loo with an external overflow be converted to internal overflow

It would mean I could loose a long run of overflow pipe

Yes, replace with the Flowmater valve which has built-in overflow as most
valves do.
Around 13 quid. BTW it means changing the lever to the supplied pushbutton.
Another way is to feed the overflow pipe to a valve made for the job and
then into the soil pipe.
In my opinion this looks messy and the valves don't seem to be able to cope
with cisterns filled by mains pressure, but you only find this out when
you're on holiday and the toilet overflows :-(