Your contract is also subject to satisfactory completion of training. Don't worry too much about the fact that you have only received an "informal" offer of employment. This is just protocol. You should get a contract on your first day of employment once most of the bureaucracy has been completed.
As far as I can remember, when you are made an offer of employment by Monarch you will be expected to attend an Administration Day approx. 2-3 weeks or so before your official start date. You will be advised what it is you will need to bring with you beforehand. You will be required to fill out details of 5 years of employment and address history. Apart from Monarch wanting to check previous employment references, this is primarily for you to get a security pass so that you can freely access those areas necessary at the airport.
Normally, you can expect to have your airport pass ready on either the first or second day of starting, I know that at the base where I am you will require this pass for you to be able to access the staff car park as well. However, there are always people that there are problems with and who have some problem or other with references. Monarch will let you know if there are any problems and ask you to chase these up.
As a word of advice, prepare all of your references. They will want exact dates with NO gaps in between. Be sure to have these ready. Moreover, you will also have to apply for your Disclosure Scotland. This is something that everyone at the airport must apply for to show they have had no criminal convictions in the past 5 years. Apply for this well in advance as it can take up to 6 weeks (but normally much quicker), and you can easily apply for this on-line as you will only require a Basic Disclosure. If you haven't already had one, then I would strongly suggest you get a yellow fever vaccination as well as this will be required of you and can be quite a difficult thing to arrange around work, and sometimes difficult to get hold of. You may well have to pay for this, but keep hold of your receipt as I think you can claim a refund.