The polling stations are open 7am-10pm, so 15 hours, before being driven to a central location for counting.
Thus, many polling stations don't have observers from the party at all - let alone the 2-4 observers needed to keep a constant watch from the first vote to the count.
[1] http://www.bbc.co.uk/news/election-2015-32229745 [2] https://en.wikipedia.org/wiki/Conservative_Party_(UK)#Member...
This is particularly noticeable at the spread-out constituencies like Na h-Eileanan an Iar, where the count has to wait for ballot boxes to arrive by boat or, if the budget is available, helicopter.
The ballot papers themselves are traceable, so if you're going to stuff the box you need to add the relevant number of ballots to the record and fake their issue to particular voters.
They're also in the custody of ballot officials; in the UK this is one area where I genuinely trust the ability of those people making up the system to operate genuinely according to the rules. We've even managed to hold fair elections in occupied Belfast.
Switching to e-voting would turn the whole thing over to known insecure consumer PCs and known suspect contractors like Diebold and Capita, all of whom I trust about as far as I can throw them; I trust the small army of ordinary people who make the paper count happen far more.
Maybe next election I'll volunteer as an observer.
The electronic solution is the one where malicious actors can substitute key parts of the process and things may be vulnerable to attacks that nobody has thought of (Meltdown/Spectre passim).
If you can pull that off you might as well say fuck it and take over the country with force.