I have the Explanatory Notes to the Scotland Act here. They state:
As I am sure my noble friend will agree, tax has always been dealt with entirely differently, not least in the exclusion of this House from consideration of tax matters since the 1911 Act. Tax is dealt with by a procedure under the Finance Act and is subject to a proper Committee stage on the Floor of the House of Commons. I believe that that is still the case. To suggest moving to a situation in which taxes can be introduced and imposed by orders-which are not amendable and which, traditionally, we do not vote against in this House-is to stretch the elastic to breaking point.