If you don't want to show on your invoice what you get charged for the Mot, whats wrong with just charging your customer the full MOT price plus vat ?
If you are a non testing station, say you get charged 30 quid, you then charge it out to your customer at full price + vat.
Just explain to your customer that if they want you to do it, you are forced by Customs and Excise to charge vat on the full amount.

Regards
Dave