Definitely wouldn't rely on street heroin in the UK but there sre some vendors on DNM's. I'm not upto date on vendors tbh but I do know that bohemia market is operating pretty well so finding a vendor there shouldn't be too hard.
There's a resource called recon on the dark net which allows you to search for products, vendors and markets by keywords and it shows you their ratings, products and feedback from all sites they're operating on. It's not exactly real time. In fact it's a bit choppy but it's better than nothing for sure. Especially if you're not already signed up to loads of markets and judt want to find one and minimise pfaff.
You will need bitcoin or monero to purchase from a dark net market. I'd say avoid clear net market for things like this. You may get it but honestly it's sketchy. Lots of weed and benzos get sold but when it comes to anything don't risk it. Even the weed and benzos are suss and often not as advertised. Clearnet sources will always be rife with people making the most of the low hanging fruit. Ironically the dark net markets are safer lol.
Whilst IV H is easy to do I'd say be very careful and think this through. There's significant risk of brain damage if you're found because you can be bought back from any dose with a simple shot carried by many service people. It's even given to addicts to carry as harm reduction. Also, its very moreish so if you do a test dose you may well find yourself putting off ctb for some time on the H and that's a dark hole to fall into. Only nice for a short period. Tolerance grows fast as does addiction.
An overdose of H isn't like getting high. You may feel high for a split moment but as the level of opiate in your sytem sky rocket you pass through nice quite quickly. Once you pass out its not necessarily horrible at all. It's just not a float into nothingness so much as a sonic boom into oblivion. You start to feel it, often before the syringe is out your arm. Your face will tingle and it feels like your lips and cheaks are kind of swollen with pins and needles. You'll taste the gear and the citric acid or vitamin C powder you used to cook it up with and then, if the dose is a fatal one you're out like a light. If it's nota fatal one and you only took enough for a recreational dose you'll drift in and out of waking dreams. Always aware of your surroundings for the most part. Your nose will itch like crazy and you'll rub your skin raw scratching and rubbing it. It's not until later on that you really feel the brunt of that. If you smoke you'll double your usual amount. If you don't smoke you'll probably want to. You'll quite likely vomit after feeling a kind of dry knot in the pit of your stomach. These are things that come with first time use and pass with subsequent use. Along with a lot of the high as you need more and more to get there and end up using ehat you can get to keep you from going into withdrawal symptoms.
With H it's like you borrow from the future with each high and end up paying the toll later down the line when the debt comes around.