Contactless £45 or less?

The POS limit is determined by the POS. For example, I was paying up to AUD$100 and CAD$100 by contactless many years ago when the limit at UK POSes was still £20.

The Starling thing AFAIK is just that you can tell them to decline contactless transactions when they are over a certain amount. This may not work at merchants which have offline authorisation for contactless (not sure if this actually exists), or for transit mode like on TfL and some buses, where they authorise a small amount then charge you whatever they want later, though there may be a lower limit for this mode anyway