Use the Eurostar website. You can book through from Cambridge to Bruxelles, using a train from Cambridge to Kings Cross (which is literally just across the road from St.Pancras International where the Eurostar starts).
Booking through protects you if the train from Cambridge is so late that you miss the Eurostar you are booked on. Don't forget that you have to check in 45 minutes before departure for the Eurostar as security and immigration controls take place at the start of the journey in each direction, and that the UK and Belgium are in different time zones (so the journey appears to take 2 hours longer going than coming back).
Use www.nationalrail.co.uk to buy a ticket from Cambridge to London Kings Cross station. Cross the road (literally) to St Pancras International. Use www.eurostar.com to buy a return ticket to Brussels - the price rises closer to the day of travel, so book as early as you can. On Arriving back at St Pancras the advice would be most accurate to know which hotel you are going to, but basically take the Tube.
Edited to note - I forgot you can book right through!
If you happen to be going to Paddington to go onto to Heathrow the next day. It will be easier to stay in one of the many hotels in the St Pancras/kings cross area and simply take the Tube direct to Heathrow (and much cheaper!)Edited: 6:14 pm, July 01, 2017
Tickets for Eurostar go on sale 6 months in advance and the cheaper tickets can sell out quickly, especially for travelling at the weekend. Leaving it this late you will pay a lot more.Edited: 1:40 pm, July 04, 2017
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