Best rail pass for England and Belgium

No pass needed in this case.
It seems you need to just buy the London to Belgium leg roundtrip months in advance to save.

A pass for that little amount of train travel will simply be a waste of money. Eurostar from London St Pancras to "Any Belgian Station" will get you to and from.

The Belgian train fares only take spare change, relatively speaking.

You know which days are which, so just save money by buying the English and cross-channel (you will not hear locals using the term "chunnel") trips well ahead. Like when flying or getting hotel rooms, buying early saves lots of money at the expense of flexibility.

Your "Any Belgian Station" station allows you a night in Brussels before continuing to Bruges/Brugge or Ghent/Gent, and if you buy a day ticket on de Lijn who operate the trams and buses in Gent and Brugge it will be valid in both cities on the same day. Such as take the tram in Gent to the Kornmarkt for breakfast then another tram to the station. In Brugge use the bus from the station into the Zentrum (centre) - the buses line up right outside the station. Use another one back, and one or several buses or trams when you get back to Gent, and they will all be on that same day ticket.

You can't do that trick with Brussels/Brussel/Bruxelles because they use a different local transportation company as they are not in Flanders.

Any seniors or young children or pets on this trip?

Thank you! Yes, one child age 6.
And we will actually be going Oxford to Ghent.
I guess I was wondering more specifically if the London Plus pass would be worth it for the Heathrow express and to-from oxford, as well as the tube. We will be in London 5 days.

The London Underground is not a train like the one you would taking to Ghent.

The Heathrow Express is a train buts it’s sole existence is travel from Heathrow to London. It’s a 15 minute ride to Paddington Station. You can save money by booking tickets for your arrival and departure dates 90 days in advance.

The train you will be riding to Paris is called the Eurostar. Again to save money book far in advance.

Other posters will need to chime in on train passes as I haven’t bought one since Nixon was in office.

From the description page of the BritRail London Plus Pass which you mention (emphasis mine)

Details:

Class of travel: First (1st) and Standard (2nd) class passes available

Discounts: Child (plus any promotional offer that might occur on this
pass)

Validity: Flexible – 3 Day, 4 Day, 8 Day (within 1 month)

Boundaries: Harwich, Kings Lynn, Huntingdon, Bedford, Long Buckby,
Stratford-upon-Avon, Worcester Foregate, Bedwyn, Salisbury, Bristol
Temple Meads and Weymouth

Exclusions: BritRail Passes are valid for travel on all National Rail
services. However, some exclusions do apply:

BritRail London Plus Passes are not valid on Eurostar services, road
links between airport and railway stations, Docklands Light Railway
(DLR), Tram links, privately owned railways, steam services, London
Underground trains and buses, boats, hovercraft, nor any other
services that are not run by National Rail.

So, no you can't use that pass on any local transportation in Oxford or London (except some attractions you get to by heavy rail such as Hampton Court Palace and Windsor Castle). No Underground (often called Tube), or bus or DLR, or any coaches.

Check out seat61.com for tons of rail tips across the UK and Europe. Eurostar tickets go on sale 6 months prior to your travel date and get MUCH more expensive the closer to your travel date that you purchase them. Once we book our airline tickets we book our Eurostar tickets. We also book our Heathrow Express tickets well in advance although not 6 months in advance. We also purchase trip insurance in case something comes up that makes us cancel arrangements, which has happened. We also don't book any hotels without a cancellation policy at no charge. BTW, we're looking at London to Brussels for next spring then onto Ghent, then maybe Bruges or Amsterdam. Seat61.com has everything you need to know about your trip.... and ours. Good luck.

So if you want to do those things with a London Plus pass you would use a day on your arrival day with the Heathrow Express. Then one day going to Oxford and another back, and connecting with the Eurostar in London although that is not covered. And back to Heathrow after you return from Europe.

So that sounds like 4 days of travel?

My research tells me a 4 day flexi is $326 US. Or $81.50 per pass day for the three of you.

Heathrow Express can be as low as $15.40 per adult weekday or $7 per adult weekend, booked up to 90 days ahead, child is free. I don't know your dates of travel but if you look at the table of fares here you can see the permutations. Even the most expensive option of an Express Saver Return (called round trip in the US) which is a walk-up fare at $49.80 per person, child free, is only $49.80 per day - the pass is $31.70 more expensive for the two Heathrow Express days, and you can do so much cheaper.

For the trip to and from Oxford, a random date for a month from now for 2 adults 1 child (child is half price), several Advance (special non-changeable non refundable ticket valid on the chosen departure only) ticket times are available for £5.40 per adult each way and half fare of £2.70 for the child, for a total investment of £27 return (around $34.40 total for both days, less than a quarter of the $163 for using the pass).

To save all that money you do need to plan, and take an hour or so of your time until you are familiar with the system, but if you can buy airline tickets you can buy train tickets.

In your case, unless you travel is in the next couple of days, I wouldn't suggest the pass as a money saver.