Advice for crits please (complete beginner)

Where do you live? There is more than one club in London

Regarding the crits, you could say group riding is not that important but at cat 4 level, it's often a big bunch moving around for an hour with a sprint at the end. Unless your comfortable riding around in close proximity to other riders, it's going to be very difficult.

The locations might not be convenient but you could try and get down to the Surrey Leagues beginner training sessions: http://www.surreyleague.co.uk/news/novice4thcattraining.htm

In terms of other venues, there's also HIllingdon, Redbridge Cycling Centre, Cyclopark and Lee Valley Velo Park. None of them are super convenient if you live in central London. There's also the Surrey League and the South East Road Racing League.

Track riding at Herne Hill or Lee Valley will undoubtable help with group riding and awareness, although it won't be quite the same as racing on a road bike. I wouldn't suggest sportives as particularly helpful for racing but from a fitness perspective it could help.

Good luck and do join a club as soon as you can. It's good fun and good practice.

A good club is really the best way to learn. A good club will teach you how to ride in a group comfortably - the etiquette and the skills.
A good club will have people who have experience with all sorts of racing - TTs, road races, crits. You will be able to learn from them.

As Gkam says, some of the top riders aren't exactly great in comparison to other pros when it comes to riding in groups. It is all a matter of relative levels though - they might not be as good as the average pro, but they are still vastly better than the average Joe. I wouldn't even think of racing (apart from triathlon) unless you are reasonably comfortable in a group. You don't have to be brilliant, but you need to make sure that you are not dangerous to the other riders around you.

Get a bit of experience in group riding with a club, then through yourself into a race and have some fun. You will almost certainly be spat out the back early on, but you won't be the only one and it should only spur you on to improve

Also, when the winter season comes along you should definitely try cyclocross. Most fun you can have in an hour on a bike

Some of the best domestic riders can't ride in groups and especially not in Crits, so you are not alone on that. I won't name any names, but there are a number in the mens and women's tour series that you will see in one of two places, off the front attacking or sitting right at the back of a group, sprinting out of each corner to get back on, depending on the race they are having.

Crits are difficult to practice, you really just have to enter and see how you go, as long as you can get a couple of laps of the track to get to know the corners, don't go pulling "chopper" moves, like slamming on the brakes in the middle of a corner because you've over cooked it, instead, roll out of it to the edge of the circuit so you don't take anyone out.

Riding in a group isn't a great help in a crit, because most courses string it out from the start to the finish with the speed and the nature of the cornering.

If you haven't ridden in a close bunch before, I would recommend doing this before racing otherwise you could be a danger to yourself and others...
The only way to learn to race tactics IMO is to actually do it, once the red mist descends its pure instinct and no amount of videos or articles will help you focus on tactics.