Hello - I found this wonderful thread having had the same immersion heater idea and having searched around. I haven't got panels yet but one installer says they always put in a separate RCD-protected consumer unit just for the inverter in order not to infringe on Part P if the existing wiring doesn't comply. A bit more material cost but seems like a good idea, and I've got a Henley block between my meter and CU anyway so it's easy for them. I have an old electric shower feed to the loft which I hope they can use; I would have thought that if it tests out OK then it would be fine, but I am not qualified so I don't know.
I don't think anyone on this thread has mentioned the case of their existing meter being analogue. I know these can theoretically go backwards but I wondered if they had some sort of ratchet to prevent it. My meter was replaced in late 2008 with another analogue one, and the installer said that it would indeed go backwards. Could I really therefore be paid three times for some of what the panels generate? I presumably won't have the trouble/fun of making an immersion heater controller as a result. I wouldn't be surprised if the supplier, when informed of the solar installation, doesn't immediately whip out the analogue meter and replace it with a digital one.