As Doodleham says, it wouldn't be safe really, they always need supervision or someone around. They are such escape artists. My best tip is keep any playpen contents well away from the sides, or they can climb on top of things and then jump over the edge of the playpen! And yes could chew their way out if left unattended.
We do advice rather than criticism on here

There are some places online where you would get an awful response for showing your cage. As you say you have given him easy access to a playpen area so it's not quite so black and white. However, generally they do better with more floorspace in the enclosure itself so they have all their regular "stuff" in one territory. Even though robos are tiny, they are incredibly active. On which note, I can't resist adding a video of my last robo Pip zooming in his savic plaza

He used every inch of it. If you were interested in one of those there are quite a few available second hand for around £35 to £40 these days. They are a bit fiddly to set up for a robo though as they're quite tall - I had 5" of cardboard inside mine so it had 10" of bedding which reduced the height a bit. Otherwise an easier option (without the height issues) is the Grosvenor XL cage - with that you could have a deep bedding end.
Grosvenor XL hamster cage
Also - if you recently bought the cage and it came from a well-known Uk pet store, some people have had a refund for it and been able to return the cage after getting a new one and the hamster moved over
Neither of those would allow for your tube access to the playpen though - but I think the Ferplast Criceti 100 would allow you to keep that ..........
It's about the only good sized cage I know of that has tube attachments. Unfortunately it is quite expensive.
Ferplast Criceti 100
Another option is the Ferplast Maxi Duna Multy - again it's not that tall but you could have a deep bedding end. Also slightly on the expensive side. Unless you can find either of those second hand on ebay maybe.
Ferplast Maxi Duna
So we would encourage upgrading the cage when you can