Can I refer you to this link.
https://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageTravel_ShowContent&id=HMCE_CL_000014&propertyType=document
Read 2.3.....and then 2.4.
If the parcel TOTAL is under £40 (and that means - not individual items) , I ask the seller to dispense with anything suggesting I bought it and to replace it with a post it note which says...HAPPY BIRTHDAY! It used to be £32.
It's worth mentioning that if you buy a second hand song sheet, it is not liable to duties/ vat, but the VAT people might still charge and it's a devil to get it back. Now, I ask the seller to write to he "commodity code" on the declaration. You can see these commodity codes on the Revenue website.