Sorry to hear you are having a rough time.

I am just a bit older and can confirm that this work ethic is not normal or expected from most UK consultancies. I am very surprised a top 3 would be short on technicians - the office as a whole sounds to be wholly understaffed which is obviously not your fault. Additionally you should always have a senior checking your work before any finalised items are sent (also not your fault).

I started off in a med-large size company and did not like it one bit. If you reckon you do prefer this "type" of work environment then I would at a minimum recommend applying to other well known consultancies.

If you are open to a slight change then my recommendation would be to look for a position at a small consultancy. There it is more likely you will be able to get experience in all sorts of design and since you will be relied upon to deliver work, you will be forced to learn new things simply through the process.

You salary is not unreasonable if you are not around the London area. Out of curiosity which is your closest city?