Are you just talking about the final holding stacks for say LHR or are you talking at any level at any hold for whatever reason?
I know Lambourne and Ockham have both had a/c in them at FL170 (17,000 ft) when holding for LHR quite regularly and possibly even Bovingdon (don't know normal upper limits), further out BRASO has held regularly upto FL240, BEWLI has held FL260, and these all in normal ops waiting for approaches at LHR (sometimes LGW in GIBSO case). I also know that BEXIL (for LGW from the south) has had an a/c holding FL350, GIBSO (out to the west) FL330 and once I saw an a/c holding FL410 at SAM. The first occasions were a few years ago during one foggy VERY early morning at LGW where a/c were waiting for the southern runway to reopen after night works as the visual approach to the northern runway was well outside limits. The last occasion was due to an emergency. We occasionally have to hold for other centres too, and a/c have held at SFD (Seaford) FL290/FL300/FL310/FL320/FL330 due to bad weather at Paris Charles De Gaulle, and something similar at DVR (Dover) when the Brussels Centre computer failed. May fixes have a published hold procedure or two, just that many aren't used in normal every day ops.