You need to ask for the letter and bank statements after the money has been in your account for 2 calendar months. Normally these can not be any older than 7 days and you need to update your bank book on the day you apply. Some immigration offices want everything on the day you apply so check the policy with your immigration office
Savings accumulated prior to 1st January 2024 are exempt from tax. Savings accumulated after this point will inevitably have an income element so will potentially have a tax liability