Under current UK Covid-19 restrictions non-essential travel, including abroad, is not permitted unless an individual has a legally permitted reason to do so.
The Home Office does not encourage travel outside the UK without your BRP card. If you choose to, you will need to make an application to apply for a single-entry visa in order to return to the UK. The fee for this is £154 and can be applied for here. This does not affect the production or validity of the BRP card or any existing grant of leave.
The cost of the single entry visa will not be refunded. If you choose to leave the UK before receiving your BRP card, you do so on the understanding that you will need to apply and pay for a single-entry visa to return. UKVI will not refund the visa fee or reimburse any other costs associated with the application.