Can electricity be sent back into the UK national grid via a normal household plug socket?
Theoretically, sure, if you have a device that produces a large enough potential. I'm a little nervous about why you would be asking...
Actually, come to think of it, most power grids probably have some sort of device to prevent the back-current from something like a lightning strike, so probably not.
In the US, anyway, the law requires that a local utility company buy power from essentially anyone who is capable of generating it and selling it to them.
Separate names with a comma.