You are asking basic Business Planning questions here.
Your gross income is the money from the product (sandwiches). You then need to estimate how much you will spend on raw material ingredients and any staff you intend to employ. Don't forget that you have to pay Employers' NI contributions as well as their pay costs. That gives you your gross profit. Then you need to assess your fixed costs. These will include rent (which you mentioned), business rates, insurance, utility bills (which you didn't mention). Take away your fixed costs from your gross profit gives you your income - namely what left for you as the owners' salary.
Only by assessing all of these aspects can you hope to work out how much you could afford on the rent.