It is essential to let the ticketing staff know of your situation when booking so that they can ensure that the seat you book is accessible.  For instance, at the Pavilion if you have walking difficulties you will need a seat in the stalls because there is no lift to the circle.  At the BIC there is a lift to the balcony, but areas of the terrace might not be easily accessible.  Most importantly, you need to be able to get out in the event of an emergency, especially bearing in mind that fire regulations require wheelchairs and walkers to be removed from the aisle and put in an appropriate place once you are seated.