- I recommend you not turn the router off at all. Turning it off on a regular basis would make the equipment at the exchange assume the line between there and you was 'poor' so would make adjustments to ensure a more reliable connection (increasing some of the settings) and the outcome would be a slowdown in your connection.
Depending on what devices you have, you could just turn off the wireless (using the router control panel via the browser, but obviously only if you are connected via a cable).
Alternatively, and something a little sneaky, would be to turn the PlusNet router wi-fi off, but you could have a separate wireless unit which could be turned off without it affecting your connection to the exchange. Search Amazon for B008NVMVNW and you'll see a device which could be plugged into the router you already have. You could put that on a timer or just unplug it whenever you don't need wireless access.
Or leave it powered up but unplug the ethernet cable... then someone trying to hack your network would spend a lot of time trying to connect to the internet whilst the only thing they were connected to would be the little adaptor.
- On any account with a data allowance, you can change one of the settings in the Plus.Net portal to limit (to zero, for example) how much you are ready to pay for 'excess' usage.
If you ever go 'just over' your 10 GB allowance, then PN has a 5 pound fee (for a further 5 GB). However (and something I think unfair) that 5 GB you have bought does not "carry over" but gets 'lost' on billing day, when all the counters for the month are reset to zero.
It has been years since I had the same sort of account, so sorry, I cannot remember where on the portal you set the limit. I set mine to 0.00 so I could never be charged extra for 'going over' the limit - with a 0 limit, they would just stop data transfers (with the exception of the PlusNet portal, to allow users to make payment should one be needed).