If you can ride a bike for 2-3 hours you'll get up Ventoux, as long as you've got the right gearing for your fitness/weight. As others have said, hill training in this country simply can't replicate one vertical mile of altitude gain over 25km without a single bit of downhill. To an extent cycling on the flat into a headwind for a couple of hours will be closer to the experience, but even then you won't be using the muscles in the same way, or be needing/wanting to stand on the pedals to push up a draggy gradient.
Whatever you do, if you can, have one or two smaller gears than you think you'll need. If you think you can get up on a 53/23, you probably won't. Not after two hours.
Having said that, Alpine-type hills aren't actually as hard as many of the ones we have in the UK. Once you've got into a rhythm, and as along as you don't go off too fast, you just get into a zone and keep the legs twiddling round. You'll get there. But do pick your day carefully. The record wind speed at the top is 200mph.