With the exchange rate between the pound sterling and the dollar rapidly moving in the direction of the dollar the Royal Mint’s move a few years ago to do all pricing in Pounds now favors US collectors.
I’ve already pushed the Darwin anniversary coin but it terms of something that has real chance to gain in value you can’t beat the newly offered Quarter Sovereign.
I haven’t seen this as a coin offered or minted before. It is a gold coin just about 1/16 of an ounce at 22 ct with the same classic design front and back and the standard full sovereign or half sovereign. If you’ve never seen one this is what it looks like:

Same design used for all size sovereigns
The glory of the British Empire is long past but if you want a chance to get something to remember it by the quarter sovereign is a solid and inexpensive choice. The mint is getting £47.95 each for them at the current exchange rate that works out to around $63 each for a gold coin in it’s first year of issue.
I grabbed 3 since that’s all I can afford, If I had a job I’d grab 10.
The web site for the Royal mint is here, or you can give them a ring at 866-519-7298.