I'm sure this has been asked before, but I have a Prestigio (Maplin brand) tablet. I've had it for 3 years, but now the battery seems to be useless. It's built in, and I can't add an external one, only an AC plug. If I charge it to 100% and switch it on, it drains itself to 2% without me even using the apps or Wi-Fi in 5 minutes! Therefore when I use it, it always has to be plugged in.

Does this mean the tablet is out of date and I should sell it/trade it in? Or can I save the battery life? It's still a very good tablet. I would appreciate all advice, even on a new tablet if I should buy one (£180 max budget).

Rechargeable batteries have a life, a certain number of charge cycles, and I'd guess yours is up. You can get iPhone and iPad batteries replaced easily but the market for them is huge.
Try Googling and Youtubing your brand and model number and "battery change", I found a few videos showing how to change the battery. But they can be tricky and some tablets break easily when you try to take them apart. Ebay might be the best place to get a new battery.