When using an OS base map you will need to purchase credit to view data below a 100m zoom. For more detail on pricing please click here.
If you have no credit in your account and zoom below 100m a message will appear
Click on Add OS Credits
Select your credit amount and click on Buy.
If you have no payment details on your account you will be prompted to add these.
Your can view your purchased credit amount by clicking on the person icon in the top right of your screen.