If you lose your BRP, you will by law have to replace this and you will need to pay for a new BRP. 

You will not be able to travel back to the UK without your BRP, this may delay or even cancel your travel plans. You may also not be able to travel to Europe or the United States until you have replaced your permit.

In addition, you can be fined up to £1,000 and made to leave the UK if you don’t apply for a replacement in time.

If you have more than three months' leave remaining, you will need to apply for a replacement BRP. You must apply for your replacement BRP within three months of the loss/theft. You can report your BRP lost/stolen online and apply for a replacement at the same time at this link. The fee for a replacement BRP is £75.20 for the standard service and £875.20 for the super priority service.

You must apply for a replacement if you have more than three months' leave remaining, even if you intend to leave the UK soon. Failure to do so may result in a fine, curtailed leave or implications for future UK visa applications.

If you have less than three months' leave remaining, you should either apply for further immigration permission (if you're eligible to do so) or exit the UK before your leave expires.

If applicable, you must also update UK Visas & Immigration with your new passport details by filling in a Migrant Change of Circumstances form.

If you are also required to register with the police, you must take your new passport and/or BRP to the police to update your Police Registration Certificate.

Biometric Residence Permit lost or stolen overseas 

If you lose your Biometric Residence Permit (BRP) overseas, there is a duty to report the loss. Even if you are not planning to return to the UK during the period of visa covered by the lost BRP, or you plan to return to the UK under a different immigration category, you must still report the loss/theft.

You must report your BRP loss to UKVI  

It will be extremely difficult to return to the UK without a BRP so if you need to  return to the UK, you must apply for a single entry visa 

More information about this is available through the following UKCISA webpage

Once you have used the single-entry visa to replace the BRP to enter the UK, you will need to apply within 1 month of your return to the UK for a replacement BRP