It means surface to 18000 feet, occasionally 25,000 feet.
There is a notam when the height increases.
To find more info, go to the AIP:https://www.aurora.nats.co.uk/htmlAIP/P ... en-GB.html
On the left, expand the tree below ENR 5 (navigation warnings) and select ENR 5.1 Prohibited and danger areas.
You'll find full information about your example.
AMC - Manageable.
Vertical Limits: OCNL notified to ALT 25000.
Activity: Ordnance, Munitions and Explosives / Para Dropping / Target Towing / Unmanned Aircraft System (VLOS/BVLOS).
Service: DACS: Plymouth Military on 124.150 MHz when open; at other times, Swanwick Mil via London Information on 124.750 MHz. DAAIS: London Information on 124.750 MHz.
Contact: Pre-flight information / Booking: Plymouth Operations, Tel: 01752-557550.
Remarks: For DACS contact Plymouth Military south of 500000N.
Danger Area Authority: HQ Navy.
Hours: Mon-Thu 0800-2359 (0700-2300), Fri 0800-1600 (0700-1500); and as activated by NOTAM.