Diplomats, as the military, aren't counted as living overseas though so that doesn't bar them from using the NHS.
TBH i think if you remembered your NI number you could probably get treated if you look and sound British/Local enough. I wouldn't try it though because the resources are for Brits in the UK.
I am also worried about not receiving the documents in time. It says on the link that @SudExpress provided, that all candidates who wish to stand must be registered 3 weeks prior to the election date. When that is finalised, they send the documents. I am really worried that given the awful length of time it takes to get any post between UK and Germany that i won't be able to vote.
To that end i have just emailed my local electoral office (the one where i will vote) to ask if i can collect them in person, as by chance I'll be in the city next month. I'll come back and let others here who might be worried know what they say.
Proxy voting, if you have someone trustworthy, is also relatively easy to organise. My dad used to vote for both me and my DH and anyone who questioned him was fobbed off with "well i have a VERY big house and pay a lot of tax, so i deserve 3 votes" and then just wander off. Git.