Use the arrow keys on the keyboard to scroll the map, or by placing your cursor on the map, hold down the left button on the mouse, and move the cursor to "drag" the map. If using a touchscreen device, simply drag the map with your finger.
To zoom in on the map, either press "+" on your keyboard, or click the "+" at the top left of the map, or click / tap on the map twice in quick succession. To zoom back out, either press "-" on your keyboard, or click the "-" at the top left of the map.
To make it easier to navigate, you may toggle on and off the visibility of the flags with the "Show Group Locations" button at the top right of the map.
Click on a flag and then the name of a group to see its contact details. The sharp flags indicate that we know here a group meets, the blurred ones just show the general area. If you update a group's details, giving the postcode of its meeting place, and then re-load the map, the flag will move to the correct place.
The map is provided by OpenStreetMap, with data (© OpenStreetMap contributors) from the Ordnance Survey for the UK, along with numerous individuals (cf Wikipedia).