There's no statutory obligation for you to obtain insurance. However you might find it wisest to seek some (although you could well find problems doing so, as many insurers would probably require you to hold some form of safety certificate for your equipment - e.g. issued by a relevant trade association - and/or a recognised qualification in fitness training).
A disclaimer, of the form you suggest, would almost certainly be invalid under the terms of Section 2 of the Unfair Contract Terms Act 197:
http://www.legislatio...reach-of-contract-etc
Chris