Healthrow/Gatwick/London to Brussels via rail: how long, and how is it?

So you're saying you don't want to spend anytime in London just land at LHR/LGW and head straight to Brussels?????
If so, (even though I love the Chunnel train)you're better off flying to Brussels since you're already at the airport. It could take over two hours to clear immigration, and take the train or tube to St Pancras station to catch the Chunnel.
If you will be touring London then the Chunnel is the only way to go. If the fastest most comfortable way to get from central London to central Brussels. Journey time is just over 2 hours, and is very comfortable. It's essential to purchase tickets in advance to get the lowest price (six months allowed):
http://tinyurl/chunnel-train

Yesterday I bought a ticket on the Eurostar train London to Brussels for $67 for May this year. I will stay in London for a few days first. I've taken the Eurostar to Paris from London many times. It is fast, comfortable, and economical when tix are bought in advance. There is a "restaurant" car that sells snacks and light meals. The advantage over flying is that the trip begins and ends in a city center which saves time and money. It is important to follow instructions about arriving at the train station at least a half hour in advance. It's easy to buy tix on the Eurostar website, then print them at home. If you are traveling by tube from LHR, it takes an hour to arrive at St Pancras.

From LGW First Capital Connect has services straight to St Pancras although the weekday rush hours are VERY crowded

It's not uncommon to have very long Immigration lines at Heathrow, and St. Pancras is hardly next-door. If you have no interest in London, I suggest you fly to BRU from home (not indicated in your profile ... ). If you know how to work the airline sites, you could even find a two-segment air trip for the same price as LHR-St. Pancras-Brussels Midi. (It doesn't cost extra, but note that you might have to carry your bags to a second train to get (one stop) to the Centraal Station in Brussels, if your hotel is nearby.) If you are actually going elsewhere in Belgium that day, make sure you pay extra for an "Any Belgian Station" Eurostar ticket. It is essential to buy the train ticket three months in advance to get the best price, and you might have to browse to a non-prime hour of the day. Allow time for outward document checks at St. Pancras Eurostar turnstiles. I once had a horrible delay (8 hours) on this Eurostar route because of a truck or pedestrian accident in France (on the way to Brussels.) But that is not a common occurrence.

Well, I can answer this better for you in a few weeks, as I'm taking the Eurostar from London to Brussels. Unless the price is significantly different between London and Brussels, I'd fly directly to Brussels. The Eurostar ticket will cost anywhere from $65-100+ one way, plus the Tube fare. The journey is about 2 hours and the trains are quite nice. I've taken it from London to Paris roundtrip in a day, and it was quite simple to board and get situated. Honestly, I fell asleep within 10 minutes of boarding and woke up as they were announcing our arrival into Paris.

All in all, I'd factor in that you will lose at least 5-6 hours for a journey from LHR to Brussels via train.