There is just something that brings out the compassion in most humans who find a baby bird out of the nest before it should be.
--- Sometimes we think a bird is a ‘baby’ when it’s actually a fledgling learning to fly. It’s easy to scoop this helpless looking novice up and carry it inside for tender loving human care. This isn’t the best thing for the bird, but it fills the human need to care for and nurture.
--- So what should you do when you find a baby bird? California State University at Bakersfield (Biology Department) recommends leaving a fully feathered (even if it’s not fully flying) bird alone. If you own a cat or dog or see other predatory animals nearby, take steps to insure the bird’s safety, ideally by removing the predator. Leaving fledgling alone with the parent you may not be seeing, is ideal. Check back often to be sure there are no predators or threats. If you feel the bird is in danger (other animals, people or autos), if you know for sure where the nest is, return it.
--- If the bird seems to be in distress or injured; or if it does not have it’s full feathers and/or you do not know where the nest is, or cannot reach it, you need to take the bird in.
--- For steps about immediate care for wildlife - here: “Since it is illegal to capture a wild bird, injured or not, a special permit must be obtained from a state or federal game warden in order to keep a young bird. These permits are issued only on the condition that the bird will be properly released as soon as it is able.”
--- Look for a rehabber in your area here:
--- For a quick, easy and homemade heat source: Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it’s not too hot.
--- Rice socks can be used on people too for muscle aches and pains.
--- According to the "California State University @ Bakersfield" FACT (facility for animal care and treatment) "If the bird seems to be in distress or injured; or if it does not have it's full feathers and/or you do not know where the nest is, or cannot reach it, you need to take the bird in." It's further stressed that in most if not all states: "Since it is illegal to capture a wild bird, injured or not, a special permit must be obtained from a state or federal game warden in order to keep a young bird. These permits are issued only on the condition that the bird will be properly released as soon as it is able."
--- In part, instructions from "FACT" with regard to what you must be prepared to do for a rescued bird include securing the immature birds in a cardboard or other small box, safe from inquisitive dogs/cats and children. It's important that the bird does not become human friendly so do not interact with it and do not allow other humans to even be seen by the bird.
--- If the bird does not have feathers or only has their first, downy feathers, maintain the temperature in the box between 80and 90 F. "FACT" recommends, "A lamp with a 40 or 60 watt bulb should provide enough heat, but don't put the lamp directly over the bird. This won't be necessary if the bird is fully feathered."
--- Make sure to cover the box with a partially light blocking, thick material like a clean bath towel. Line the box with shredded white paper towels (the less print on them the better) or newspaper, again, the less color print on the pages, the better. These are two highly heat retaining materials easy to change when soiled. It's important to avoid any drafts.
--- Thank you for caring enough to rescue this baby and good luck! ---
Footnote: Yes I know you're in the UK - but these guidelines do apply internationally