Is this an IMS tank for the KLR? If so, call IMS and ask them to forward ship it for you. Let them pack it and ship it using their logos and packing methods. Go with USPS. This will be the cheapest by far. UPS and FedEx are way expensive, I never, ever use them. Anyone who ships for a living will know this.
I've sent all sorts of MC parts worldwide using USPS including fuel tanks, Carbs, Stators, heads, headlights, body parts, panniers and dozens of other weird things. I always label them as used cycle parts and use a non descript term when possible, like "used cycle part". With the fuel tank I never state "fuel tank", but Cycle parts. Fuel tank will immediately get a red flag. Once the clerk accepts it at your local USPS it should go unopened until it arrives in UK.
When I sent a stock fuel tank I cleaned inside with Alcohol to remove petrol smell, finished up with soap and hot water. Then left it bake in the Sun for a few days. I shipped it with fuel cap and petcock OFF. Any hint of odor and they will confiscate it.
The other possibility is to sell the tank in the USA. Better than paying shipping it. Get another tank for the KLR in the UK or EU. Thing is, if you sell in US, you'll still have to mail to someone. This is possible, trust me.
Shipping USPS to UK with insurance should run about $80 USD. I sent one to Australia for about that price. (from California) Unless your KLR tank is really huge and a weird shape. If it requires a odd sized box, then it's gonna cost you more, and will get more scrutiny.