common to do post grad at same uni as UG ?

1) Do many people do their post grad at the same uni?

2) Are you more likely to get accepted because they already know you?

3) Could it be that the others passed, but this uni was the only one they could get into ? or that they really like the uni (I don't really have any other uni to compare it to)

4) I would've thought that if you did well (2:1/1st) you could easily get into a higher ranking uni for post grad studies ?

It's pretty common. Most unis offer a discount for undergrads who stay on for postgrad, as an incentive.

From what I've seen, yes. This especially applies if you have a borderline undergrad result. If you have a 2:2 but it's at the higher end of the range and the staff who know you feel that you are capable of achieving better, then you have an improved chance of acceptance. Unis where you aren't known, may well filter out any application below a 2:1 without further consideration.

Either option is possible. It's equally possible that the uni is simply the best place to follow their personal research interests. I did my Masters at a different uni but went back to my undergrad uni for my PhD, as that's where the expertise and primary research material was available. However I did find it useful to experience a different university as the theoretical approaches to my subject were very different.

At postgrad level, it's more common to follow your research interests, regardless of perceived status or ranking table placements. For example, my Masters uni was RG - I did a taught Masters in a rather niche subject and that was the only uni in the UK which offered it. My undergrad/PhD uni is a former poly. Oxbridge would have been useless for me at both Masters and PhD level, as the relevant departments don't have a focus on what I want to research.

1) Do many people do their post grad at the same uni?

2) Are you more likely to get accepted because they already know you? I didn't like to ask what grade they got in their undergrad except the one applying to Cambridge.

3) Could it be that the others passed, but this uni was the only one they could get into ? or that they really like the uni (I don't really have any other uni to compare it to)

4) I would've thought that if you did well (2:1/1st) you could easily get into a higher ranking uni for post grad studies ?

At postgrad level, it's more common to follow your research interests, regardless of perceived status or ranking table placements. For example, my Masters uni was RG - I did a taught Masters in a rather niche subject and that was the only uni in the UK which offered it. My undergrad/PhD uni is a former poly. Oxbridge would have been useless for me at both Masters and PhD level, as the relevant departments don't have a focus on what I want to research.

This isnt aimed at you specifically because I have no idea what field you are in, but if no universities in the UK are interested in your area except a niche ex-poly, then you are possibly going to struggle with career prospects after your PhD (at least when it comes to getting an academic job) since there is presumably only one university in the whole country who would want to hire people that work in this area.

"Following your research interests' is one thing, but realistically most people have fairly bendable interests (especially since undergrads dont really know enough to know what they are interested in beyond very broad terms), and imo you should factor career prospects into your decision somewhere too. Becoming an expert in an area that noone cares about might be a fun way to spend 3 years, but it may not be a strong foundation on which to build an academic career afterwards. Academic job prospects are bad enough, without closing off the majority of universities in the country.