Very little preparation is needed for a tandem skydive - they are convenient and fairly well priced if you just want a sample of the sport - you are simply strapped to an instructor and they do the hard work. The cost is in the region of £220 or £300 with video.
Be aware that a tandem skydive is like being in the passenger-seat of a fast car. All the thrill but none of the skill. Moreover, tandem jumps do not count towards progression. Although tandem wingsuit jumps have been attempted (bit.ly/tandemwingsuit), they are ineffective due to the mass:surface area ratio and potentially high risk.
Perhaps a better option than a tandem is an AFF level 1 jump - it costs around £350 but you are taught to skydive, it counts towards progression (you can continue if you wish) and you jump out of the plane with two extremely experienced instructors. In terms of value for money, this option is exponentially better.
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