LSE no longer has a midday auction.
The only intraday auctions on the LSE are those on the SETSqx segment. Orders for SETSqx symbols will rest in the order book to be included in the daily Auctions at 8:00, 9:00, 11:00, 14:00 and 16:35 GMT.
IBKR routes orders to the electronic book ("Auto Book") on the LSE for the SETSqx trading service. Your SETSqx stock order can trade during these periodic auctions. Note there may not be any trades during these periodic auctions unless there is a matching order to trade the other side of your order. If your order is the only order in the Auto Book there will be no trades during that auction.
In other words, you simply have to enter your order on an SETSqx symbol and it will be eligible to take place in the auction. To search for a stock's Security Information on the LSE web site click here.
Find more information on the SETSqx page of the LSE website.
Additional Information