Many of those ~10 million will simply not vote, others will vote for other parties and some have died.
Turnout in European parliament elections in the UK is typically on the low side, in 2014 it was only 35.6%. In contrast the turnout in the 2016 referendum was 72%. While turnout may be higher for these elections than in 2014, it will still most likely be well below the level seen in the referendum.
We will, however, have to wait for an exit poll to get a reasonable idea of how turnout in today's vote varies between leave and remain voters in 2016. The pre-election opinion polls are predicting a fairly large range of results due mainly to difficulties in predicting turnout.
There will also be a non-zero number of leave voters who vote for other parties today, particularly the Tories, but also Labour and even some for explicitly pro-remain parties (Lib Dems, Greens, Change UK). In Scotland and Wales voters also have the option of the SNP and Plaid respectively. Finally, in Northern Ireland the political system is almost totally distinct to the rest of the UK and voters there chose from a separate set of parties. Again, we really need to wait for exit polls to see how many 2016 leave voters vote for each party as the pre-election polls are somewhat scattered.
Finally, ~0.5million people die in the UK each year (so ~1.5 million since the referendum). Given the demographic of leave voters skews old, a significant number of leave voters will have passed away.