[Britwrench]

I would send it by USPS air parcel post unless you want to spend money or the parts are needed quickly.
You will need to fill in the larger customs form. If you are "creative" the parcel will be delivered fairly quickly, usually up to three weeks. I assume that these parts are a "gift".
For your information, "Billingshurst" is the town and "West Sussex" is the county Funnily enough, Billingshurst used to have a Porsche dealership in the town center.

If you are sending by UPS, then the "invoice" is one you produce and they will need three copies. Doesn't have to be anything fancy, something you run up on the computer will do. Nothing else is needed. Obviously we have to supply company invoices for customs checking as we send parts to Cyprus and places like that.
Call UPS and see what their best price and delivery times are first.