I would assume that under a certain threshold, HMRC doesn't even want to hear from you, because extracting taxes from you costs them more money than you are going to pay. On the other hand, I cannot find anything written about that subject.
I'd suggest to call them at 0300 200 3300 and ask them. Have the annual cost of the server ready, and tell them that you will stop asking for donations if say 6 months of cost is covered. There may be an official threshold that I was unable to find.
Obviously if you receive £1000 in donations and spend £100 for the server, they will want tax payment. If its £110 in donations and £100 for the server, they will likely not care. If they tell you to register as self-employed, it's not difficult, just a bit of a pain. In that case you'd have to pay tax on your income (donations) minus cost (cost for the server and any other cost).