In my experience with the London Stock Exchange you would get hammered. Whenever I have placed a offer for a share, it does not appear on the spread.
Example: let's say the spread is 9.0p - 9.2p so I place a order at 9.1p. In Canada, my order would appear instantly and someone could snap it up. However, my experience in London is that the trade wont get executed until the bid exceeds my offer price. Same for sells... So you are effectively buying and selling at the wrong end of bid and ask.
Now this is only my experience with the LSE (not just PCI) and I trade with TD Waterhouse, and others may have different experiences.
Add Stamp Duty, Commission and the hassle of organising your tax returns and I v much doubt it is worthwhile.
BUT the biggest negative from my perspective is being able to actually profit from it in terms of timing and discipline etc. A bad trade could send you into a panic.... This is why I am a buy and hold investor!