They are strict... They want to know how much you spend on pretty much everything. Exactly how much you earn. Everything written on your payslips must be accounted for (pension, student loans, tax breaks for company cars, etc). Every ounce of debt you have.
My advice is to go through your bank accounts and record exactly how much you spend on each individual bill.
rent, gas, electric, tv licence, contents insurance, council tax, phone, internet, mobile phones, gym memberships, pension contributions, total monthly petrol costs, if it costs you money each month you add it to the list and write it down.
You will need to estimate how much each of these will be at your new home (if it all goes to plan). Mostly it will be the same, but if you think you can live without a few extras (like sky tv with all the trimmings, etc) then feel free to remove it from the list of expenses
You may also need to give a figure for how much you spend on groceries each month. I'd recommend giving the minimum amount you can manage on every month.
Then you will need to find a website with a affordability calculator (most banks and building societies that do mortgages have these, some are much better than others!) such as Nationwide and Money Saving Expert and put all the figures into the calculator. It will then give you an amount that they would consider lending you.
I would also advise signing up for the 1 month free trial (and cancelling it before the free month ends!) with Experian and Equifax to see your credit report and try to improve it before you apply.
Massively essential thing to do is get yourself on the electoral roll. It gives a huge boost to your credit score and many banks are really strict about this and won't give you a mortgage, or won't give you the best rates if you aren't on the electoral roll.