On the page about the archive it says
"Please note that the Online Archive does not allow you to download and read issues offline. If you have a Digital or Print + Digital Subscription you can download your subscription issue by following the instructions at http://www.model-engineer.co.uk/digitalaccess."
To me it sounds like you can access the archive online, and can only download new issues as they are published.
I wanted some back issues, I have the print only subscription and contacted them to upgrade it. I gave up, first call they told me I could only do it at renewal time, and if i wanted it now i would have to buy a second subscription.
Second call to them, I can upgrade, but have to cancel my existing subscription. I was not totally convinced I would get a refund. Plus as my wife bought the subscription using paypal, I would have to do the whole lot by email.
A lot of faf, and knowing my luck I would either end up with no refund or missing a few issues.
I am now just buying them on ebay.
What issues do you want I have a few duplicate issues.