Thanks to everyone that offered feedback and advice.
For anyone still interested, the managers did not approach the meeting as a disciplinary matter. One of the managers was my line manager (I will refer to him as Manager 1 and the other as Manager 2) and privately I fully expect that he would have been making a case to treat it as a disciplinary matter but would have been convinced otherwise by other senior management.
It was he who referred to my actions as "childish" etc..
As recommended in several answers above, I apologised from the outset, acknowledging that, regardless of my standing in the company, it was utterly incorrect to arbitrarily decide that I could leave the premises to work elsewhere.
To clarify on a couple of points, where some answers have had to make assumptions due to lack of information:
In working from home I was still contactable by mobile and email (I was logging on to our server remotely to perform the work).
My standing in the company can possibly best be described as a middle-senior position, without the title (and, to some extent, the pay, but that is not a major grievance for me). I have expert and referent power but little authority. For example I assign work programmes/schedules across different operational departments, with jobs prioritised according to their commercial value to the business. This involves disappointing colleagues in sales who want their five small orders processed first because they were ordered first, irrespective of the fact that doing those jobs would delay jobs that are literally 50x higher in value. This is always a difficult conversation but I think it is a concept that they are gradually getting to grips with. I do not have the authority to challenge/discipline the relevant departments if they go off-programme, but they follow my programmes based on my referent power: they see management trusting me with high-value tasks and me delivering demonstrably positive results consistently. Therefore even if the schedule doesn't immediately appear to be 'fair' to individual customers, they still run with it (in the main).
In my view it is primarily down to (2) that I will have 'escaped' this becoming a disciplinary matter.
There have been some answers that have suggested what I did was OK and others suggesting that leaving the place of work would always be wrong. The bulk of the responses, which I fully agree with by the way, say that the rules and/or culture dictate what is acceptable: I absolutely failed here, that is clear.
Nonetheless, Manager 2 accepted culpability for assigning too many tasks in a short period of time. Manager 1 was aware of the additional tasks assigned by Manager 2, but did not accept any responsibility for allowing them to accumulate without considering the need for slack or additional resources.
When I made the point that I was being assigned multiple 'urgent' projects by two different senior managers and needed a consensus as to how to prioritise, Manager 1 rolled his eyes: Manager 2 stepped in to respond before I could ask what Manager 1 objected to about that point. I consider it to have been an absolutely reasonable statement. (NB - Manager 2 is senior to Manager 1 in the company hierarchy.)
Manager 2 suggested that we devise a simple program to re-establish suitable timescales and what resources are needed: positive, although this should have taken place at the outset. I WAS asked when the projects were assigned to me what I needed to get the projects done and I answered "space", specifically referencing it being unrealistic to be left on my own in the office as the only person taking spam calls, customer enquiries etc. and simultaneously expect good productivity.
You may ask whether my seniority in the company is really as great as I think it is, if those circumstances are commonplace and the answer is yes, it really is. However we have 5 admin/sales support staff in an office of a $0.8m annual turnover business and me frequently on my own in the office of a $5m annual t/o business. Both businesses operate on different phone lines and it will be another 6 months before they are consolidated onto one system.
I was asked whether I was considering leaving the company once I had finished my qualification. (Once the phone conversation earlier this week escalated, I made the mistake (yes, another one) of saying that that is what people close to me had strongly advised me to do.)
I responded to say that is something that I would need to think about over the weekend and that we should reconvene early next week to discuss it. I could have faux-confidently said "no!" but I don't want to burn my bridges with the place, and if I answered "no" only to hand my notice in in 2 or 3 months then it would not go down very well at all.
So I am going to review the situation today and go back to them with some constructive suggestions next week.
I have probably made it blindingly obvious to you that I feel that the root of the problem (the 'problem' being my relationship with the company at this time) is Manager 1: his expectations, management style and approach to providing support and resources (i.e. not doing so) but I will need to be careful how I address that issue.
Regards to all,