Firstly, I would suggest that unless your export shipment is over �10k, it may not be worth using them.
The credit is issued by the importers bank and it alows a UK bank to pay on its behalf if the exporter presents the correct information (the "Letter" plus shipping documents etc) within a time scale. This guarantees payment, and you could, in theory, rest easy knowing you will be paid.
They are the most expensive and complicated payment procedure but they are also the most secure- the only thing better would be payment up front.
HOWEVER:
I don't understand your comment about the banks as they banks LOVE Letters of Credit - they make a fortune out of them in charges as many things can go wrong and the banks will charge at every opportunity. Your documents will be scrutinised and even the tiniest error or spelling mistake will mean the bank won't pay up until you have corrected the change (and paid for it) - and this is all done to a time limit. The figures state that 80% are rejected at the first presentation (at about �75 a time for the banks)
If your payment goes through companies that are not technologically advanced, it is easy to see where errors can be made in hand writing or translation.
Use them for big shipments or where you have new customers and you are not sure about them and need a safeguard although they will go wrong with new customers all the time as they won't be used to them. Also, don't forget to factor in the costs of the Letters of credit into your prices