First Proper Tour Advice Please...

When have you booked it for? Its only January now, so plenty of time to train as long as you start pretty soon.

If you are fairly fit and able to cycle 92 miles in a day, its a good start, especially as you mention cycling only 37-55 miles per day on that tour.

If I were you I would aim to do at least 12 hilly 40-50 mile rides before you go, and draw up your plan now!

Overall i think you'll be fine, but some hilly rides wouldnt go amiss.

I'd look carefully at the route profiles. In some ways the amount of climbing is almost more important than the mileage. A lot also depends on how heavily-loaded you are - eg if you are carrying full camping equipment or staying in hotels/hostels.

I'm not especially fit (I tend to plod away). My entirely subjective assessment is that 60kms with 1100 metres of climbing is relatively straightforward while 90 kms with 1700 metres of climbing is a fairly big day. And 60 kms with 1700 metres is somewhere in between.

It's probably worth considering fallback plans so you have a Plan B in your pocket - eg do you have the option of a rest day? (IME it's probably always worth having a day in hand) if there's a big day can you split it if need be? Unless you are travelling in high season (eg July or August) you don't need to book all your accommodation in advance (although again if you are planning to stay somewhere where there's only one small hotel and no other accommodation nearby then it might well be worth a phone call the previous day).

There's nothing in the UK that really can compare with a long alpine climb - Snowdon is 1000 metres (or so) and I suspect that you'll find very few climbs in the UK with more than say 300metres in altitude. However, even though I'm used to long climbs, one of the hardest days recently was in Dorset - all those little climbs add up. so I'd think of the long alpine climbs as a series of short climbs linked together - with a long cruise once you get to the top. My experience of French roads is that it's unusual to find really steep bits - usually it's a question of trundling up gradients of 5 per cent or so.

Make sure you have enough water-carrying capacity and something warm to wear at the top and on the way down.

The climbs in France are basically uncomparable with those in the UK, think 15 miles of cycling up hill at around 6%. The closest one near me to compare them with would be Hartside pass - about 5 miles at 5%. I would say the French climbs are 'easier' in a way, but obviously you are going at them for 1hr+ so you need the stamina. (im not saying Alpine climbs are easy - they certainly are not!)

IIRC on the first day of "The Way of the Roses" ride that we recently did, we did the same amount of climbing in 92 miles as we did in one ascent of Ventoux (15miles)

Don't forget that you only have to pedal for half the day. Much easier than a tour of flatlands.
Trick is allow loads of time 1 pass a day is plenty. Expect to be on the climb for 2-3 hours depending how much load you have.
If you want the closest thing to alps in uk try hartside pass, melmerby to alston and back 5 miles of ascent each way.
Or possibly ballahulish to rannoch moor
Busy with motorbikes in summer though.
If it's hot you'll have to carry water, not many shops on them thar hills!!
Whilst in Bourg d'Oisans, Alpe d'Huez is almost compulsory. also Lautaret & Galibier worth a day trip.
Once bitten by the col bagging you will want more.

I used to live in hilly Hastings so was forever going up and down. I found that big Alpine and Pyrenean climbs were fairly easy compared to the steep hills we have here. The major routes are all well graded (they were military routes designed for mules) but some of the minor one are a lot harder.
Fitness on the flats doesn't translate well to climbing ability but it doesn't hurt. The only training for climbing is climbing.
Make sure you have gears suited to your strength, fitness and ambition and that you ride at your own pace. In a professional race, a climb is hard because you have to race it. Touring cyclists of all shapes and ages ride those same climbs with loaded panniers but at a much lower gear and slower pace.