Just like High Stret banks, LC&F use your money to make loans to businesses and then make their profits, and pay you interest, from the repayments made by those businesses.
However there's a BIG difference between an investor's relationship with a High Street bank and one with HC&F. If a High Street bank lends your money to a business, and that business defaults on the repayments, the bank still has to pay you your interest and return your capital. If HC&F lends your money to a business, and that business defaults on the repayments (possibly because the business has gone bust), you lose the lot.
http://forums.moneysavingexpert.com/showthread.php?t=5346049