Hello,
If you ask BT to provide you with a quotation for broadband they should be able to indicate if sufficient copper pairs exist for an additional line to be installed.
You will need to budget for line rental and broadband costs for the contract duration plus, perhaps, installation costs. There are great deals on broadband at the moment which include free installation and reduced or free broadband costs for a specific time-period.
Do look for bonded broadband that does not tie you to a specific ISP. This gives you choice in terms of cost and provides some resilience against failure in a particular ISP's network.
Also, do ensure you look at bonded broadband as opposed to broadband load sharing solutions, they are very different!
Hope this helps.
Phil
www.velocomms.com