If you wish to work in the UK, you will need a National Insurance number in order to make sure you are taxed at the correct rate.
Once you have your FLR(M) BRP card you can apply for a National Insurance number - since you have FLR(M) rather than a spousal visa you will likely need to attend an interview, and then it can take around a month to receive it.
In theory, you can start working before you have an NI number, but employers usually ask for it in the application process, and you will be taxed at a higher rate until you have it, so it's best to get it sorted sooner rather than later.
See here for how to apply: https://www.gov.uk/apply-national-insurance-number
Sent from my iPhone using Tapatalk