All caves on the island are open - no gates or access conditions to worry about. A little discretion and good manners are all that is required. Park sensibly, respect peoples property, do not enter working quarries and simply have a good time. Caving isn't all that popular so encountering inquisitive locals or climbers is fairly common.
Most of the access paths are used regularly and maintained by the local climbing community. The island has many hundreds of bolted sport climbing routes, some of which pass conveniently close to the cave entrances providing suitable protection to aid access. Typically these are resin anchors. If you use these consider donating to the Portland Bolt Fund.
Well this is easy - there are no caving clubs. Historically there were a couple of Dorset based clubs who were active on Portland as well as else where but these no longer exist. Even in those days there was never a club hut on the island.
There is a bunk house at Portland Bill (Portland Bunkhouse), another at Victoria Square (The Bunker) and the YMCA at Reforne. There are also a couple of very basic campsites on the isle open in the summer months however plenty of others to choose from in Weymouth.
On the whole the caves are warm; boiler suits and grots being favoured over furries and oversuits. PVC suits or wetsuits should be avoided - you will be too hot and the caves will eat them. Do not forget your knee pads - these are essential unless you enjoy sore knees. Quite a few of the caves are reasonably clean, however there are some which are notoriously brown on the West side of the island. Portland mud is extremely sticky, caustic in nature, has a delightful smell and a tendency to stain. Any metal work which comes in contact will rust if not cleaned immediately after use. Best to consider the stuff 50:50 mud / cement blended with a bit of sea salt for flavour!
There are few pitches within the caves and where these exist most can be free climbed by the competent. Alternatively a single 10 m ladder and suitable life line will serve you well. Some of the caves contain bolts; these should be treated with extreme caution as they are of varying vintages, not maintained and have not been installed under any formal scheme. You must make your own judgement whether these are safe to use. Natural anchors are usually plentiful in the form of wedged boulders in rifts but there's a good chance these are loose if thoroughly inspected. Getting to some of the entrances can require good lead climbing ability or SRT. Details can be found under each cave.
There is no shop / hire for forgotten kit. Great Western Camping in Dorchester has a limited supply of climbing equipment but you'll not find a caving suit etc. within 50 miles. There is a Screwfix in Weymouth and although perhaps not your typical outlet for caving equipment does sell most of what you need for Portland; Helmet, headtorch, knee pads, boiler suit & gloves!
There is no Portland cave rescue. Any emergency call would go to Dorset police who might eventually contact Mendip Cave Rescue. Mendip is around 2 hours away so do not expect help quickly. Your best bet is to self rescue to the entrance if at all possible. From there help will be more likely as scooping up broken climbers from the cliffs is frequently rehearsed by the Coastguard. Please make sure somebody knows where you are. The local climbers know the cliff lines intimately including where many of the entrances are; however few venture in.