From a purely legal point of view the householder is allowed to buy and have an export meter installed by anyone competent. (They can own their own import meter, technically, too.)
But different electricity suppliers have different policies. Some only want a combined import/export meter so this must be fitted via the supplier and DNO. I'm not sure that all these policies meet the letter of the law.
Also there is the time delay in registering the export meter, which the supplier has to do. For some unknown reason they state that this can take up to six months - which is just crazy. Only 50% deemed can get paid in the interim.
This, of course, can't possibly be connected to the fact that suppliers make a bigger profit out of deeming.
The whole metering side of the electricity business is in need of a good shake out.