I've been hit by unexpected data roaming charges previously, so when I travelled recently I made sure I switched mobile data off before I left the UK. All good.
However, when I switched my phone on after installing an overseas sim (so both SIMs installed), I ran up 11MB of mobile data on the Plusnet SIM even though mobile data was still switched off. This of course blew my cap.
Now, it may be a peculiarity of my particular phone (if it started using data before checking whether it was allowed to) or it could be a generic Android "feature". Anyway, I recommend anyone travelling to a non roam like home destination to set their smart cap to £0 before you leave the UK.