That seems far too long, even for customs. I ordered my wedding invitations from an etsy seller. She posted them from the US (east coast) on 18 March. But in her online receipt, she provided the USPS tracking number for me. When I hadn't received them by earlier this week, I looked up the tracking info and found that they were in the UK. I phoned ParcelForce yesterday, paid the customs due over the phone, and the invitations were delivered to me today. ParcelForce did send me a letter like the one Victoria received, but I beat them to the punch and phoned them a few hours before the letter even reached me.
Ten weeks, though? That's not right. I agree that you need the tracking number. Once you have it, you'll be able to check online and find out if your parcel is in the UK or not. If it is, in all likelihood it's being delivered by ParcelForce. So get in touch with them and see what's going on. When I phoned them, they were very helpful and were able to find my parcel through the USPS tracking number.