I hold a 32 page UK passport which is nearly full. Page 4 is a complete unused page and on page 26 I have half a page unused.

Unfortunately, I wil not have the time in which to renew my passport as my current visa will soon expire, knowing that it takes 10 working days to renew at the British Embassy.

Considering that I have only one and a half pages left, could anybody advise me whether I should either do a border run to Cambodia (whole page for cambodian visa), Penang (whole page for tourist visa) or be safe and just go on a Mae Sai border run?

Whatever option is decided, I will then have sufficient time in which to renew my passport.

Advice would be appreciated.

It sounds like you should have no problem making a border run to Cambodia. As long as the pages left are visa pages and not endorsement or some other term noted on them.

You could also get a 7 day extension at immigration (1900 baht) which might be enough time to get a new passport.

No need for a whole wasted page with a Cambodian visa. See http://evisa.mfaic.gov.kh/e-visa/vsindex.aspx and get an e-visa which you print out yourself and staple it to your passport, discard when back in Toyland! Easy!!