Britcoin matches bids and offers differently to how most exchanges do it. If you post a bid at MtGox, for example, it is first matched against the lowest-priced offer, then works upwards in price from there if more offers are needed to satisfy your bid.
At Britcoin, it seems to work the other way. If you post a bid, it is first matched against any offer at the same price, then works downwards in price if more offers are needed to satisfy your bid. Or at least, it's something like that.
So I think what can happen is that you can bid £100 (for, say, 0.01 BTC) and if there is an offer at £100 then a trade will take place and it will show up on the chart. Similarly, if you offer 0.01 BTC at £0.25 and there is a bid at that price, a trade will take place and it will show up on the chart. But this does not mean that the order book between the high and low prices was ever "worked through".
Oh, and the way you enter bids and offers at Britcoin is "unusual" too. You type a number of BTC and a number of pounds (instead of a rate). So people might occasionally make a stupid mistake.