Glasgow, Liverpool and Bristol are not (how do you define "major"?).
Liverpool has, what, 5-6 million passengers per year. Certainly not major.
Though defining major is a question. Arguably, it depends on a lot of different things, but I'd say you'd be looking at 20+ million passengers per annum to be major.
There are 30 airports in Europe which meet that criteria including four in the UK (Heathrow, Gatwick, Manchester and Stansted). All four UK ones have a rail link
Looking at the top 30 in Europe, Dublin (34.6 million passengers in 2024) is the largest without a rail link. Now, Britain is bad at infrastructure, but the Republic of Ireland is somehow even worse. Though one is now planned as the first post indicated. Still, it's likely not going to be operational for close to a decade.
Palma is the second largest without a rail link. The issue they have is Palma is highly seasonal and a significant number of tourists are heading off on coaches to the myriad of beach resorts on the island. Still, one is also planned and is could be open by 2032. But the vast majority of 20 million passenger plus airports have a rail link.
Alternatively, it'd be interesting to find out the smallest airport served by rail and the smallest served by a dedicated rail link.
Olsztyn Airport in Poland served 142k passengers in 2023 and that has a dedicated rail link spur. I think there are six trains a week which serve the airport with some, but not all flights, linked to a service.
In the UK, we have Teeside Airport Railway Station which serves Teeside International. Teeside Airport served 228k passengers last year. Though the train station is closed (or 'temporarily' closed); and the airport is well over a mile from the station which hardly makes it an airport station in my opinion despite the name.
The smallest in the UK is probably Southampton Airport (631k in 2024) which has Southampton Airport Parkway just next door.