Hi, great points put forward by paul. I am wondering if you are talking about or have considered the fire proof insulation board and similar such as fireproof plasterboard etc? These would simply be joined end on end with upright studwork framing (CE grade 3x2) Without knowing the area size, it is difficult to comprehend the costs given. There are many ways to comply with the fire regs especially given the materials available now. Although Standard T&G would be cheaper than most materials as the cost per 8x4 foot area is around £30. We must all appreciate that most grades of The&G will present large dead knots that will clearly be an issue unless filled with heat resistant filler. Where in any doubt I would search for further quotes from reputable companies as they will accept liability for the install and should present a certificate stating that it was installed, fit for purpose. Additionally to this, you may find alternative methods that could be extremely cost saving.