Your yellow GB Machin was issued on 17th August 1993. I do not have Scott number. Printed by Enschede it should have two phosphor bands. You need to check because there is a less likely other possibility. (Harrison, Coil)
The stamp on the left is part of the long-running Machin series.
If you’re using the Scott catalogue as a reference tool, the Machin stamps have their own numbering system starting with the MH prefix. You’ll find these stamps listed after the “regular” Great Britain postal issues.