In a way I initially tried what you said but didnt seem to work. I saw someone was wanting 1 BTC for £3.50. I went to the home page and put up an offer matching this but nothing really happened so after a few minutes I cancelled it.
I've had this happen to me but not with such whole numbers before. For amounts with lots of figures after the decimal place I have to divide the BTC by the GBP to calculate the rate and then place an order ever so slightly better than that rate (and than the quoted 'Cost / BTC').
After struggling for a while I just thought sod it I'll just put my bit coin up for £4 and see what happens (prior to doing this I looked at what peopel were wanting for GBP but no one was after a 1 BTC and none at exactly £4). Literally seconds later when I went back to my account I saw I had £4 sitting there instead of 1 BTC. Someone had accepted my offer.
Disclaimer: The following is my current understanding of the system and is probably completely wrong.
It is possible for one order to partially fill another. For example, if someone places an order to buy 10 BTC for 40 GBP the system will calculate a 'Cost / BTC' of 4 and add this entry to the book. If someone else comes along and places an order to buy 4 GBP for 1 BTC the system will again calculate the 'Cost / BTC' and place the order in the orderbook. However, it will not be long before the system realises that there is a bid and ask offer at the same rate and a transaction will take place. The 4 GBP for 1 BTC order will be filled and the existing 10 BTC for 40 GBP order will be changed to an order for 9 BTC for 36 GBP.
This is all well and good for simple examples like this but things get a bit hairy when the best bid and best sell rates overlap.
If instead, the second order is to buy 3.50 GBP for 1 BTC then I believe that this order will also be filled automatically and that the original order of 10 BTC for 40 GBP will be modified to 9 BTC for 36.50 GBP (rather than to 9 BTC for 36 GBP as I'd expect). Also, I don't think the 'Cost / BTC' is recalculated and so will remain at 4.00. This is also problematic because a third person sees this offer and places an order to buy the 36.50 GBP for 9 BTC and the system calculates a 'Cost / BTC' for this third order of 36.50 GBP / 9 BTC = 4.06 (2.d.p). Being larger than 4.00, no trade takes place.
By the way: How do you guys stop your e-mail address from appearing on your posts? I've tried selecting 'hide my e-mail address' in my profile (no change) and I've tried deleting my e-mail address from my profile (not allowed).