if it is possible for someone to determine how they voted
If the poll is conducted the same way as the recent elections in UK then the answer is, potentially, yes!
Have a look at page 12 (bullet 1, Stage 2) on this (Electoral Commission)
link.
Write the elector number (including the polling district reference letter(s) if not pre-printed) on the corresponding number list next to the number of the ballot paper to be issued
Therefore, in theory (and with my tin foil hat on), if someone was to have access to the ballot papers (which are manually counted.....) and the polling station lists they could be cross referenced back to the individual elector number and therefore the individual elector.
I questioned at the last local election here why this was done (connect ballot number to elector number) but the polling officer was unable (or unwilling) to answer my question.