There is not a forumula as such, however you may find the following guidance links of use: 
Tax when you get a pension: What’s taxed - GOV.UK
National Insurance and tax after State Pension age: Overview - GOV.UK
Thanks

Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal.

You are free to use any of the three rate HMRC provides, or you can use a rate obtained elsewhere, such as a newspaper or the London Stock Exchange.

For your convenience, there are exchange rates at:

The National Archives: Exchange rates from HMRC in CSV and XML format

For older rates at:

The National Archives: Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009

You are free to use any of the supplied rates or one of your own choosing

You need to register for Self-Assessment if your income from savings and investments is over £10,000 in the financial year.

You can find further information here:

https://www.gov.uk/check-if-you-need-tax-return

If you earn interest over your allowance but under £10,000 and are employed or receive a pension, HMRC will change your tax code so you pay the tax due automatically.

The code for the current year will be based on how much interest you had in the previous year.

Further information can be found here: https://www.gov.uk/apply-tax-free-interest-on-savings

When paid through PAYE, it is an employer’s duty to consider the tax implications when making payment awards to their employees and ensure PAYE is accounted for correctly.

It is the employer’s responsibility to operate PAYE correctly and in doing so must consider when relevant payments made to employee’s should be subject to PAYE. Sections 18 and 686 of the Income Tax, Earnings and Pensions Act 2003 and Section 6 of the Social Security Contributions and Benefits Act 1992 determine the point in time when earnings are chargeable to tax and National Insurance contributions, and subject to PAYE:

Earnings are charged to tax on the earliest of the payment or entitlement to payment. Entitlement is not necessarily the same as the date on which an employee acquires a right to be paid, but the date on which they are first entitled to receive the payment.

For example, if you start employment on 5th April, but are not entitled to be paid until the last working day of that month. The earnings will be charged at the tax rate applicable on the day the payroll payment is made. 

Hi KSDZ91
HMRC is unable to provide specific or tailored advice throught the Online Community Forums and cannot comment on any form of scenario as we may not be in possession of all the relevant information.
In this instance it will depend on the amount of interest earnt, for example:

If you earn interest over your allowance but under £10,000 and are employed or receive a pension, HMRC will change your tax code so you pay the tax due automatically.

The code for the current year will be based on how much interest you had in the previous year.

Further information can be found here: https://www.gov.uk/apply-tax-free-interest-on-savings

Your options if you exceed your allowances are also covered here.

Hi Danny 
Please use the guidance here: Tax on dividends: Check if you have to pay tax on dividends - GOV.UK
 

This would be dependent on your bank. If you find you are unable to cash your cheque abroad, then we can send a replacement cheque to a nominee, who you appoint and who has a UK bank account. 
Contact us by webchat or phone via https://www.gov.uk/government/organisations/hm-revenue-customs/contact/income-tax-enquiries-for-individuals-pensioners-and-employees to allow us to do this.

Please see the guidance Tax on foreign income: Reporting your foreign income - GOV.UK

The commercial software and service suppliers listed in this guidance have given evidence that they’ve developed software or manage a service (or both), that can produce one or more elements of a Company Tax Return: 
Commercial software suppliers for Corporation Tax - GOV.UK

HMRC is unable to provide specific or tailored advice through the Online Community Forums and cannot comment on any form of calculation, example or scenario as we may not be in possession of all the relevant information. Please use the link below to see if you need to register for self assessment and send a tax return.

Check if you need to send a Self Assessment tax return - GOV.UK