Yes, NASGP is approved by HMRC as a professional organisation, and you can claim tax relief on your membership subscriptions. This GOV.UK page tells you more (we’re listed under ‘S’ as ‘Sessional GPs National Association of’).
Any booking fees are also tax deductible, as they’re an expense incurred in doing your job. Your accountant can advise you on how to classify these in your individual case.