I used to live in Pimlico and fly from Biggin for a few years, so am familiar with your problem...
A 1.5 hour drive is par for the course, quite short, in fact. Longer if you want to drive out there on a Friday afternoon... even when driving on a Sunday morning with no-one around I never managed less than 1 1/4 hour, if I remember correctly. And it's a pain, so I only did it a few times.
To get to Biggin I took the train to Bromley South and then a taxi from the station. There is a taxi rank at the station, and while not all taxi drivers are familiar with the south side of the airfied, you can always direct them. I allowed 1 hour for this trip, including the 10 minute walk to Victoria, and normally made it with 5-10 minutes to spare.
To get back from Biggin I took a local minicab firm [local to Biggin, that is, NOT to Bromley] and made sure on booking they knew where to go. However, advance booking was necessary, just calling them up would give you a 20 minute wait, and much longer if they are busy.
I was flying from the flying schools right at the entrance of the airfield estate, so it was easy to find. If you are hangared all the way around, for example in Hangar 504 or in the old Air Touring hangars, it might be more difficult.
There was a minicab firm present in the terminal that knew their way around, but they were very expensive, and without pre-booking weren't any faster than the locals.
So, in a nutshell, I would say no problem getting TO Biggin, but getting back needs an arrangement with a local minicab firm.