Although this does not answer your question about London specifically, the procedure in the U.S. is covered in the 7110.65, 4-2-5 b.:
"NOTE-
Restating previously issued altitude to "maintain" is an amended clearance. If altitude to "maintain" is changed or restated, whether prior to departure or while airborne, and previously issued altitude restrictions are omitted, altitude restrictions are canceled, including DP/FMSP/STAR altitude restrictions if any."
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