Where the current goes strictly follows ohms law so if multiple loads are present it will be apportioned according to impedance, e.g. lowest gets the most!
Insofar as meters you had best check the details of your meter online to find out what it's reaction will be, I would be suprised if you had an electro-mechanical one, most are electronic these days but many are not bi-directional and may indeed flag up fraud if there is any reverse current flow.
If you are considering a GTI you should speak to your DNO (Distribution Network Operator) (not who you pay bills too) who you report power cuts too. They will advise you if your contemplated GTI is acceptable or not.
Finally you cannot obtain credits for the power in the UK unless the whole installation and installer person meets the renewable certification requirements (MCS).
As for knowing when you are exporting some meters change there LED color so you could potentially detect that or it's a current clamp and voltage tap (for that I use an AC wallwart) and some software to determine the phase relationship between voltage and current, ignoring power factor this swaps 180degrees between import and export.
Have fun and play safe