27.2mm Seat Post Reamer, Beg/Borrow/Hire/Buy

[Hotgolf]

Adjustable reamer usually, as 27.2 isn't an off the shelf size.
If you're going to do it yourself just be careful, and use plenty of cutting fluid, something like trefolex. It'll give a cleaner nicer looking cut.

 

[letmetalktomark]

Argos cycles wanted £25 to do mine.

Seems like a bargain considering the faff if it goes wrong....

 

[Reluctant]

Best solution is to fit the correct size seatpost.
I really can't see any bike shop hiring you a tool to do this, there's just too many ways to do the job wrong - not sure I'd even agree to do it if asked.
"Bike" in Clifton have an adjustable reamer and I'd guess ZeroG on Park st have too. Or Argos as the others have said.