Nick.
Depending on your ceiling joist spacing one layer of 12.5mm BG Fireline should meet the requirements for 30 mins. Two layers would give 60 but unless this is a separating ceiling (between premises or demise) the English req is 30. The white book is quite good at this stuff and I often include the white book reference when writing up a job spec. You have to make sure you comply with the spacing, have perimeter battens and noggins. If you you use the 15mm board the joist spacing and noggin requirements are a bit more generous. Stick to the system, right fixings, fixing schedule and all the right finishing materials exactly as the product certification. Avoid the Siniat option, they specify their own huge screws. Siniat has a 60 min cert for one layer of 15mm but I tend to avoid putting it in a spec because contractors will weasel their way to save every penny they can and may not follow the rather exacting certification details.
On some jobs in old buildings we spec for a grid of Gypliner hung off the joists. It ensures you can fulfil the spacing requirements. Old buildings often have irregular joist set out and levels can be all over the place. Also spaced for 1800 x 900 boards as the access to basements can be tricky for adult size boards. Sound reduction is improved too.