In Brussels allow plenty of time to change trains. You will go through customs and immigration for the UK just like an airport. I only allowed 30 minutes and had to do the OJ dash to make the train to London. Arrive in London at St Pancras train station. Just walk over to Kings Cross train station and buy your ticket to Cambridge. They leave every hour. When I did this a few years ago our company credit card did not have the micro chip to buy tickets from the self kiosks. But the new cards should work just fine. Or book your tickets online and print at the hotel. It was a worth while trip, but it is a long day of trains.