http://www.caa.co.uk/docs/33/CAP493Part1.pdf
7.1 When an amendment is made to a clearance the new clearance shall be read in full to the pilot and shall automatically cancel any previous clearance.
Controllers must be aware, therefore, that if the original clearance included a restriction, e.g. “cross ABCFL150 or below” then the issue of a revised clearance automatically cancels the earlier restriction, unless it is reiterated with the revised clearance.