Personally, I wouldn't recommend Manchester. It is an incredibly competitive and difficult university to get into, with ludicrous entry requirements for a mediocre CS course. It has potentially more competition than Oxbridge, on the basis of having higher entry requirements. Most likely, that is because of students living in Manchester primarily applying to that university (since Manchester is a massive place, lots of students living in Manchester apply). So, unless you live in Manchester, I wouldn't recommend firming or insuring that, unless there's a specific reason to go there.
The best university out of the three for CS is clearly UCL. That is the best out of the three remaining choices. So, I would firm that and snatch that if you received and meet the offer. UCL are one of the best universities in the country for several areas of CS, so if you're interested in further study, that would be ideal as well.
So, as your insurance, you could pick either Warwick or Durham. Given that you feel better about Warwick, I would consider Warwick. However, I would also consider making Durham your insurance instead, as it has an offer rate of AAB, which is much lower than Warwick's AAA. Chances are, if you don't meet A*AB for UCL, you probably wouldn't meet AAA for Warwick either. Therefore, on the basis of being safe, I would recommend UCL as firm and Durham as Insurance, unless you are confident you will obtain AAA.
The course from Warwick and Durham are both pretty standard and nothing special for a CS course. So, either one is best. Personally, I would go for Durham insurance just in case you don't get AAA, but if you are confident that you will get it, then you should go Warwick as your insurance.
In conclusion, I cannot tell you which course based on the factors like campus and student life. But I can tell you based on which CS course content is better, that UCL > Warwick = Durham > Manchester.