Making assumptions based on starting with a 77D, which should in itself be fine for getting into birding, I would say that your viable options are going to be somewhat limited due to the cost of quality glass. Don't fall for being tempted to buy cheap though as quality optics will be at least as expensive and probably more so than your camera itself. However the quality glass will make a bigger difference than the camera. Now for some suggestions:
If you are just planning to work with relatively close in subjects, such as on your own feeder or in your own backyard garden, then you might be able to be happy with a good fixed 300mm lens or a zoom that maxes out at that. Using Canon glass on Canon bodies is always going to maximize the quality, but Sigma and Tamron also make excellent lenses with Canon EF mounts and will be considerably less expensive. I don't know the retailers for the UK, but you should also be able to find reputable dealers with used lenses.
Especially if you are planning to try for subjects like the water birds where you often cannot get really close, or just a general lens that would cover near or far, you can't beat the Canon 100-400mm L series zooms. The latest mkii version is probably way more than you want to spend, but you can still find used mki versions. Also I can vouch for the Sigma 150-600mm Contemporary lens which would be much more affordable both new and used. The Canon 100-400 in either version is a much better lens, but also much more expensive. Tamron also has a similar offering that I can't personal speak for, but I have seen good reviews of.