Answer for your specific connection:
I assume due to the much longer layover of 1:41h for your desired connection, it is usually not shown because there are connections with a shorter layover available for the same arrival time.
One general trick to avoid high-speed trains is to add an via station to your itinerary, which is not served by the high-speed train. Unfortunately on the NS International website there is only on the itinerary planner the possibility to add an via station (and it doesn't provide a price for your specific connection).
But you can book the Rotterdam-London connection on the website of the Belgian Railways and there is also the option to add an via station. I chose Mechelen as the via station, because it is not served by Thalys/Eurostar but the IC stops there. Currently the prices start from 64€:
Original and general answer:
Dutch Railways (NS) provides a separate booking site where you can book international connections. It is also possible to book such a connection on the SNCB website.
There they also offer the slower connections with IC trains between Rotterdam and Brussels:
However keep in mind, that prices are calculated based e.g. on the occupancy rate of the trains and the closer you book to your travel date, the higher are (usually) the prices. It's even possible that a rare situation arises where the slower connection is more expensive than the travel with the Thalys between Rotterdam and Brussels.