Y ou don't need to go to 10,000' to cross the Alps. The Gt St Bernard between Aosta and Sion is 8100 tho Sion was a hideous ripoff last time I went through - handling mandatory due to a 50m walk across the apron! Can't imagine either how passports come into it if the route is Italy - France but maybe being a B47 there's a 30year time-warp involved too.
The most direct route is via the Gt St Bernard, about 650 miles to London, 12hrs realistically. The coastal route another 150, perhaps a bit less.
Weather is indeed the major factor esp in something as slow as that - very careful planning and met availability (with regard to winds aloft over the Alpine stages) will be needed. The Alps aren't a small undertaking in a turbine helo, let alone a clockwork mouse. The low coastal route is likely to be costly as suggested but there must be little club fields en route, those are the ones to use but you'll have to research whether helos are allowed and if fuel is available to visitors, plus how to pay. Don't assume credit cards work at club fields. Avoid places like Cannes unless you're made of money. The Mistral can blow for days at a time and will reduce a B47s groundspeed to between 30mph (if blowing gently) and zero.
The first thing you'll need is set of charts and a Guide Delage to identify your fuel stops in France plus equivalent guides/info for Italy and Switzerland if you think of going that way. Then you'll have some idea how to do it.
Sounds like fun. Need a nav?