In the London Stock Exchange, only thinly traded stocks (those that are not part of the FTSE-100)are traded on the SEAQ. It is a quote-driven market made by specialized and competing dealers also known as market-makers.
The system contains no public limit orderbook and as such the system is not MiFID compliant for this reason the LSE developed the SETqx system which allows members of the public to display limit orders although these orders are traded through so that typically incoming market priced orders will trade with the Market Makers Quotes and not at the better prices which may be available inside the market makers bid/ask spread.
SEAQ effectively gives a monopoly over all of the incoming orderflow to the market makers since members of the public cannot submit limit orders which can match with each other (because of the lack of a proper orderbook). hence all deals on SEAQ have to be done at the market makers prices
bid/ask spreads and hence trading costs on SEAQ are typically very high due to the anti-competitive market structure.