Unfortunately I no longer have any 3/4" collets. The style of dead length collet used by Britan comes under the DIN6343 standard. The 1-1/4" collet does not appear to a standard size, and I expect the 3/4" collet is the same. The following company claims to cover Britan collets, although I have no personal experience:
Collets
Changing to an ER collet would cripple the machine in terms of use. If I needed a special size collet I'd make one to match the existing design. I have the means to grind collets, but wouldn't bother for home use.
In general capstan lathes are not much better than a boat anchor without tooling. Before I did anything else I'd make darn sure I had all the parts to make a functional machine. All the switch gear on the headstock is missing. As is the lever for high and low speed, if the machine is so equipped. It looks like the machine has an autolock on the cross slide. The toolrest seems to be missing. More importantly in the righthand end of the machine is there a pull out plate with an auxiliary motor that runs coolant, hydraulic and pneumatic pumps?
A picture of all the parts spread out would help identification.
Andrew