Hello there. I've had to do this with a few of our hamsters. I did a lot of research as the first time I couldn't come to terms with it at all. Now I think it's the kindest thing you can do when they are suffering.
Not all vets do use gas anaesthesia but my vet says they won't do anything else because they feel it's kinder (and so do I).
It is exactly the same as if they have surgery. It's just anaesthetic gas so they go to sleep. Not only are they asleep but it means they are also anaesthetised so they don't feel pain. I think it's the kindest way. They just go to sleep and and don't know anything about the injection.
Yes the downside is you can't be there if they have gas. But I personally wouldn't have the injection done without anaesthetic (and there are vets who will do this - it's more vet policy than a legal thing).
In terms of them going into a tank. I don't worry about that. I have a snooze cube - a fleece hide - that they all love. They happily walk into it and snuggle down to feel safe, when in the vet examining room, and the vet takes that and puts the snooze cube down in the tank. Our hamsters have usually been pretty weak at that stage as well and not interested in anything other than being safe in the snooze cube. Our last but one syrian could only just manage to get into it.
So they are in a nice, familiar cosy, enclosed fleecy house and just go to sleep. If the hamster did suddenly decide to climb out of the snooze cube they would just be in an empty tank - just like being in a treatment room for examination. But they would go to sleep so quick they wouldn't have more than a few seconds to even explore it.
Hamsters are tiny things and it only takes the tiniest bit of anaesthetic gas to make them go to sleep. In fact it's why surgery has to be so carefully worked out so they don't give them too much anaesthetic.
The other thing I found out in my research, which I like, is that many many times, the hamster actually passes under the anaesthetic gas before the injection. Because they are usually weak and unwell. And the dose would be a bit higher than for surgery.
The vets still have to give the injection because their protocol says they have to, to ensure that actual death has occurred (in case the hamster didn't quite pass under the anaesthesia and came round later - which would be awful.
I think sometimes imagining it makes it worse than it is - it's not like some kind of torture gas chamber - it's just the normal anaesthesia process like they do when doing surgery.
I noticed you're in the north of England. So am I. Although that's a very big area! But if you message me I can tell you who our vet is.
I didn't have a vet the first time and rang round a few and was horrified to find most of them don't give gas first - even if you wanted it. And some are very blase about hamsters. It sounds like you have a good vet.
I'll link the snooze cube - all our hamsters have loved them - it's plenty big enough and just has a small entrance. There's another small entrance/exit at the side in one of the seams. I always close that one off with a bulldog clip first so they don't climb out the side while the vet is carrying them in the cube. Easy enough to put your hand over the front.
Even though you can't go with them, they usually give you some time to say goodbye in the examining room. I find usually I've already done my goodbyes on the way there and I'm more concerned with the hamster feeling secure being handed over to another person so I just talk normally to them and say - you're just going with this nice lady for a bit. They don't know they're not coming back. But that's just me.
Hamsters do know when they're ready to go as well and sometimes they get to the stage where you've left it too late and it's an emergency and they are screaming in pain.
Metacam can only relieve pain so much - it's not enough for cancer pain.
We had a robo with cancerous lumps that got bigger. It was too much of his body area to operate. It also affected him being able to walk. I changed his cage so he could manage and he continued to eat and be active but looked awful he had lost so much weight. He then started to chew at one of the lumps and the vet told me I needed to let him go. She said when they chew at the lump, they're trying to amputate it because of the pain. And they lose weight, even if they're eating, because they're just feeding the cancer.
It's so unfair, but when they get older, they often do get something wrong with them.
Maybe you have something similar to the snooze cube?
I have the two on the top right of this page. It's the Homer mini ones - they have loads of space inside for a syrian (or a dwarf) and it fits in my guinea pig sized pet carrier. The standard ones for rats are far too big!
I use it for taking them to the vets generally as it's easy to lift them in and out of the pet carrier in the snooze cube. Providing you've bulldog clipped the back exit or they can jump out the back.
Fuzzbutt "Homer" mini hammocks and cubes for mice, hammies, suger gliders