Assuming that it's the non-STC'd version...
For a major mod application, it needs to go direct to CAA Gatwick but via an "approved company", there aren't all that many of those - but if you are really keen contact me privately and I can probably help.
I don't have the charge scheme to hand, but it's of the order £400 minimum charge which gets you 3 man-hours of effort from CAA. If your company is good, that should be enough, if not, expect to pay around £130/hr after the first 3 hours.
The submission will need to include design details (drawings, etc.) and a compliance report with the relevant certification standard (probably BCAR Section K in this case) and test reports (assuming it's been tested by someone somewhere, or that testing was carried out under the applying company's flight-test approvals).
It's likely that CAA will have some questions about this, so expect a few weeks delay and a few hundred quid in fees whilst this is sorted out with your consulting approved company. Then they'll issue a "Draft Airworthiness Approval Note" allowing somebody from the CAA regional office to come and inspect the items. They are generally only checking that the bits are the same as were declared in the original application.
Once that's through, the full "Airworthiness Approval Note" or AAN is issued. This gives you full approval - it's likely only to be a one-off for your aeroplane alone UNLESS the supplier of bits had a JAA production approval (POA) in which case it could be a generic approval for all of type.
That's the position right now, and frankly it's not worth the hassle since there's nowhere to use skis in the UK. If there were, buying a cub on a permit, and putting the application through the PFA will save you several thousand quid and a lot of grief.
In theory, once EASA happens in the next year or two, a French approval will become an EASA approval and should then be accepted automatically in the UK. Whether it'll really happen this way is frankly anybody's guess at the moment.
Seriously however, if you really want to do something like this to an aircraft with a UK CofA, drop me a line and I can probably sort you out via an approved company of my close acquaintance. It will cost you a lot of money, but it is possible. If it's on a PFA permit, you might find it still pays to hire a suitable consultant, but you'll miss out on the CAA fees.
G