tourist re-entry for return flight to Canada

[markeeil]

Hi I'm sorry if this question has already been dealt with here, I didn't find it and I'm not entirely sure how to phrase it, but:

I am currently in the UK (recently arrived at Heathrow, I have my return flight to Toronto in June), but I am going to Spain in the meantime, the idea is to return to UK shortly before my flight back home.

I heard that the 6-month tourist visa stamps are single-entry only and I might be denied entry when I try to re-enter the UK. Anyone have any advice about this?

Thank you very much
Mark