If you cannot provide access to your energy meter (due to a locked box, dog, or other obstructions) your energy supplier has the right, under current legislation, to estimate your bill. This means they could possibly under or over estimate your bill. If your bill is under estimated, you could be sent a high 'catch up' bill once an accurate meter reading is taken.
If your bill is overestimated, you will be faced with paying a high bill for more energy than you actually used until an accurate meter reading is taken. To avoid this, you should contact your supplier to make other arrangements (eg. reading the meter yourself ). However, under current legislation your energy supplier must use its best endeavour to read your meter once a year.