As far as I know. there was no hard and fast rule about ratchets. However, as soon as your PV is installed, you can expect the meter to be replaced, as your supplier may have a separate export tariff. That will be lower than the import tariff (as you’d expect) and they won’t want the meter to go backwards and effectively give you the same rate in both directions.
I wrote this only days ago - it happened in the days of FIT.
There’s been a lot here recently about automating the charging process, so there’s a lot of reading ahead for you.