Yep, it is impossible to CRH in the UK being as no coin older than 1971 circulates because the 5p and 10p were reduced in size (same size as the old shillings and Florins).
Metal detecting is certainly an option, but check out the laws, Europe in particular has bizarre laws when it comes to metal detecting.
It is fairly easy to get burnt when it comes to UK coins as an American because it is a whole different mindset. First, condition is paramount, Spinks (the Red Book of British coinage) doesn't even list prices for grades less than fine. What is considered "old" over there is completely different than over here in the US.
There are 6 main periods of British coinage:
Celtic coins
Roman coins (often overlaps with Celtic coins)
Hammered coins (ranges from the departure of the Romans until midway through Charles II Reign)
Early milled coins (ranges from mid-Charles II up until the start of modern milled coins with the advent of steam powered minting equipment in the later 1700s)
Later milled coins (ranges from the later 1700s up until decimalization in 1971)
Decimal coinage (1971-present)
There are also different grading standards with British coins, very few dealers will use the Sheldon scale (see
CoinsGB-Grading for details)
I'd suggest picking up a copy of Spinks (The Standard Catalog of English coins), due to the pound/USD conversion rate it is a pricy book, but if you want to do anything with British coins it is essential (see
http://www.amazon.com/Coins-England-United-Kingdom-Catalogue/dp/1907427198/?tag=treasurenet01-20 ).
If you've got any British coin questions feel free to shoot me a PM since that is primarily what I've been collecting.