There is generally high availability for the lowest tiers on connections including the 1943 Köln - Brussels and 0625 Brussels - Köln - you can often get these fares with less than a month to go. For connections with do not require an overnight stop in Brussels, there is often availability, but it's very limited. When the fare from London to Köln is over 59 € (2nd class/Standard Class), then availability within Germany affects the onward price, so the fare offered could be 69 € to Köln but 109 € to somewhere much further away, but when it's 59 € to Köln, it's always 59 €. I presume this is the same with 109 € First Class tickets.