The generation meter is only recording generation, not export. Traditionally they would assume that in a doemstic environment, 50% of the generation has been exported, but that was a short term compromise to get the whole solar PV scheme of the ground properly, it was always intended to meter it properly at a later date.
Your feed in tarrif will comprise a per unit rate for everything you generate (to encourage you to have them installed in the first place, it could be as high as 43p! depending on when you had it installed) you also get a separate rate for everything you give back to the grid (which is likely to be less than you have to pay the grid to buy it from them). Then of course for every unit you inport from the grid, you have a unit charge the same as the rest of us
The problem with your meter running backwards, is if for every unit you take from the grid, you pay them x, and for every unit you give back they give you 0.8x, with the counter running backwards, it simply looks like you have used less from the grid, than you have, they don't know how much has gone each way. The generation meter doesn't help as they are prepared to base how much they pay you for exporting on the basis of 50% of total generation, but not your main electricity charge (which has to be accurate)
So they are required to change your meter, the good news, is that it does not have to be a smart meter, many of the later mechanical meters had a backstop so it wouldn't turn backwards, so it would correctly log the import and ignore the export. All the later digital meters AFAIK do this too, a popular model displays "rEd" (reverse energy detected" when this happens and doesn't decrease the register.
I do not know if there is such a thing as an import/export meter for domestic available which does not have smart meter capability, there is no reason one could not exist, but I would expect they wouldn't be widely available as they want to encourage smart meter take up.
So, they can insist on changing your meter to one that doesnt run backwards (and indeed they should), but you can insist that it is not a smart meter (and there are possibly good reasons why you should), but if you do do, its unlikely they would be able to fit one with an export register (unlikely to be an issue to you, unless you thing you are exporting more than 50% of whats generated)