Train from Heathrow to Southhampton

Heathrow doesn’t have a mainline train station so you would always need to change somewhere.

However you can travel directly from Heathrow to Southampton via National Express Coach (bus), which is the option I’d choose.

Hi Jane! We are doing the same thing in September (Heathrow to cruise port). The National Express bus is direct -- so we booked a ticket with them. We got a flexible ticket in case we want to take an earlier or later bus. They have buses every two hours I think. https://www.nationalexpress.com/en
If you still want to take a train, I'll leave it to someone else on here to give you advice on best route and schedule! :)

I think the rail option would involve either the Railair Bus or the Elizabeth Line train from Heathrow to Reading then change for train to Southampton but while it might be a very little quicker than the bus, for convenience I’d take the Nat Express direct bus every time.

The best 'rail' route is the rail/air bus to Woking Station then train.

It is not the cheapest, but is the smoothest.

There is engineering work on at Wimbledon and service is thinned out, but trains are running.

I've made this transfer from Heathrow to Southampton twice and the National Express Bus is the way to go! Quick, efficient, comfortable, the least expensive, and involves the least amount of luggage handling. You can pay for a flex fare so worries about flight delays are not a concern and it does run every two hours.

At Heathrow:
If you arrive at terminals 2 or 3, go to the Central Coach Station to get your National Express Bus. Follow the signs it is easy to find. The departure board will tell you which stand to go to.

If you arrive at terminal 5, it is very easy and quick. When you leave the arrivals hall, walk across the terminal to the exit door, cross the street, and you are in bus area. National Express stands are in the first row on the far left. I think they are 11-15, you can expect this bus around 14 or 15. I think it was 4 minutes from the time we left arrivals until we were at the bus stand.

At Southampton:
You can get a taxi from the station to your hotel for less than 10 pounds. Our hotel , Premier Inn West Quay, was close enough and the weather good enough, that we just walked it in about 5-7 minutes.