Ok, you folks gave me some great tips when I first posted about this 2 months ago. Isn31c and Nigel were especially helpful. I followed their advice and booked seats on EMT train from St Pancras to Luton Airport, but today got an email saying there would be no trains running that day because of a strike by the train drivers' union. I was able to get a refund of the full amount, with no fuss. But, we still need to get to the airport.
So, back to the drawing board!
I'm looking for the easiest, least complicated (and preferably economical) way to get from our hotel, which is about a block from the Russell Square tube station, to Luton Airport. Taxis are always a possibility; I see minicabride.com offering a ride for about £90 and up, depending on vehicle chosen. A transport service is similarly priced; Rome2Rio shows taxis averaging £400, which seems unlikely for a 30 mile drive.
We are comfortable taking the tube or bus, if anyone has a suggestion for those means. We will have luggage, but carry on only.
Thanks for your help, once again!