Straight from the Royal Mail website,
"Receiving mail from abroad
All mail that enter the United Kingdom may be examined by customs, but normally letters, postcards and packets containing only documents or similar forms of correspondence are excluded.
If you are receiving a package from overseas (unless it’s from another EU Country), please be aware that it may incur customs charges. Any package assessed as being liable for customs charges will also incur a Royal Mail handling fee of £8.
Any goods imported into the UK over the value of £15 are liable to import VAT. Gifts between private individuals over the value of £36 are also liable for VAT. Goods and gifts over these values may also be liable for customs duty. You no longer have to pay customs duty for goods up to the value of £135, however you will still be required to pay import VAT and excise duty where applicable.
Please note that these limits do not apply to alcohol and tobacco goods where both excise duty and import VAT may be charged. It is not permitted to send alcohol and tobacco products into the UK from another EU country unless prior arrangements have been made to account for the excise duty.
For more details please read the Royal Mail guide to receiving goods from overseas."
Royal Mail does NOT open you packet, it is examined by Customs.
However, this whole thing is about Fedex.