I could be wrong, but I thought under the Sale of Goods act the retailer has the obligation to repair or replace the goods, at their discretion, if they are deemed to be faulty. The act, if memory serves me correctly, allows for a reasonable time for the retailer to undertake the necessary action.
In that reasonable time, I think it would be acceptable for retailer to consult an expert on the problem. In the case of Rock Shox, that expert is Fishers.
It is probably also worth noting that if the bike shop were to replace the upper assembly of the fork, it would most likely take more than a week for them to get the parts and for the mechanic to swap all the parts from the original fork. It is quite likely that the mechanic will be undertaking the task for the first time.
The alternative is to allow the shop to send the fork to Fishers who will most likely deem the item faulty and use their trained technicians to replace the offending part. The fork will probably be back to you in under a week.
I would recommend that you insist that the shop send the forks recorded delivery.
The fork crown is not a sufficiently large part of the whole bike for the shop to be obliged to refund the total cost of the bike, or even the price of the fork.