The Greater Manchester Police website denies that the Islander is currently in use. Instead, it states that they're using a BN 4000 Defender as their fixed wing aircraft:
http://www.gmp.police.uk/mainsite/pages/asu.ht m
Either way, the aircraft carries basically the same equipment as their helicopter:
http://www.gmp.police.uk/mainsite/pages/asuequ ip.htm
The advantages of fixed wing aircraft (compared with helicopters) are that they're cheaper to buy, cheaper to fuel, cheaper to maintain, easier to fly, slightly faster and able to reach greater heights. The disadvantages, of course, are that can't hover and they need a runway. (Wikipedia states that the fixed wing aircraft is scheduled to move to Barton aerodrome - alongside the helicopter - if, and when, a runway can be provided. Light aircraft can often operate with a grass runway so that might not be quite as difficult as it sounds).
Chris