Assuming you will be carrying out some structural alterations, sanitary or drainage alterations or a material alteration to the means of escape in some way the scheme will require Building Regs approval. Part of this is a statutory consultation between Buiding Control and the local Fire Authority. They are also consulted by the Licensing Department of the local authority. They will see the plans and recommend anything they see as needed if it hasn't already been put on the plans. The only time you might not need this is if you take over an existing premises where there is no change of use and you make no alterations.
As for numbers, yes the Fire Authorities have always done this in the past (up until a year or so ago). Nowadays it can be down to your fire risk assessment. I've just done the calculations for the club I was dealing with for 784 people over 3 large rooms. The Fire Safety Officer will want to see the risk assessment and check it over in respect of maximum permitted numbers. If he doesn't agree he/she may well revise them for you. Without getting too technical it all depends on what happens in each area. Occupancy or floor space factors can reduce your numbers considerably (and your ability to generate revenue) if you decide part of the premises will be a lounge for example. One square metre per person will be applied over the floor area. For other parts used for dancing .5 of a square metre can be applied and even up to .3 sq.m. per person around bar areas. After that you've got to ensure you have sufficient exit widths to get those numbers of persons out in an emergency.
Contact me nearer the time. If you can send me a plan with the measurements and floor areas on it I can give you a rough guide as to the maximum numbers you are likely to achieve.
As for the guidance applicable to your type of premises - click on the link. This is the guide for small and medium places of assembly for up to 300 persons. If you anticipate over 300 then use the guide for large places of assembly. These are used by everyone in fire safety.
Don't forget though, if it's going through Building Regs then you'll need Approved Document B1 and not the above guide.