I don't think it's possible to convert an LLP to a Limited Company. I think you'll need to cease trade of the partnership and transfer any assets to a newly incorporated limited company.
If they want to keep the same name as the LLP then they would need to incorporate with a different name, dissolve the LLP and then change the name of the company to the original name.
You could look at selling goodwill as part of the transfer of the business - it depends a little on the circumstances, but you might be able to justify a price that generates minimal/no capital gain for the members of LLP and a tax deductible expense in the LTD..