As you are in business class you may use the Galleries Arrivals or any of the three Galleries Club lounges in terminal 5. The breakfast is better in Galleries Arrivals but to get there you will need to clear UK immigration, which you may not be eligible to do based on your nationality.

You will be able to have spa treatments also in the lounges.

Finally, your business class boarding pass entitles you to £15 worth of goods from Starbucks in terminal 5C, but must be done in one single transaction.