What's all this talk about boxes dying?
99% of TiVo problems are hard-drive related. Normally, unless it gets hit by lightning, or trodden on by an elephant, any problems can be fixed by a new hard drive - providing an excellent oportunity to upgrade your capacity at the same time
If it does get hit by lightning, then TiVo CS will do you an out-of-warranty swap-out for about £90, and transfer your lifetime sub to the new box for free
If you're lucky enought to have bought your TiVo from someone who never subscribed it, you might even still get 1 year's FREE swap-out warranty from TiVo - as this runs from the date the box is first registered with them.
If Humax (or another) bring out a new Series 2 TiVo in the UK, it's possible that TiVo will do what they did in the US and allow you, for a limited period, to transfer your Lifetime Sub to one of the new machines.
Even if they don't, you will still get back most of the value of the lifetime sub if you sell the box on eBay in a year's time - so even if you only keep it for a year, then you will have saved, effectively, £120 compared to the cost of a monthly sub
Edit: Changed "allow you to transfer your Lifetime Sub to one of the new machines for a limited period" to "allow you, for a limited period, to transfer your Lifetime Sub"