Wormers are available to buy both over the counter or online here in the UK but a "qualified person" has to oversee it.... it used to have to be a pharmacist. When I buy my horse wormers online they usually ask how many equines you have and ages and sometimes sizes and if they are pregnant or lactating.
For what it is worth, I am in the same camp as
@centrarchid . I had a rooster that I saw poop out an adult round worm. I did nothing for a year and saw nothing more. His health deteriorated and I eventually culled him. He had a diseased heart but he did not have a heavy worm burden. I think I found one in his gut. All the hens in the pen with him are still alive and kicking a year later and laying well with no wormer administered. Some are 4 years old. I am sure they will not be worm free but it is my opinion that a healthy bird's immune system and perhaps even the parasite's presence itself, prevents heavy infestation and that birds that get overloaded may well be due to a compromised immune system or other underlying issue..... I am talking round worms here though. I have no experience with tapeworms so far.
I also had a pullet that was struggling with health issues (Marek's) poop out a whole writhing mass of worms..... it was horrific! She clearly had an overload I believe due to her compromised immune system. I have no idea how or why her body expelled them without a dewormer being applied. She eventually lost her fight to Marek's, but the flock she was penned with continue to lay well a year later with no obvious worm burden and no wormer administered.
I used to routinely worm my horses and now I have faecal worm egg counts and have had consistently low readings for the past 8 ish years. I was worming once a year for tapeworms but there is now a saliva test for that too, so I can hopefully stop putting chemicals into their bodies that they don't need. This all adds to my opinion that animals are able to tolerate a low worm burden without it automatically getting out of hand despite the thousands of eggs that adult worms can shed.
I necropsy birds that die whenever possible and check the gut of those surplus cockerels that are butchered as well and I have only found the odd roundworm inside them, even though some of them have a compromised immune system due to Marek's Disease.
It may be that there is something environmental at play..... we have a mostly cool, damp climate, although we are in the middle of a scorcher at the moment. I try not to introduce new animals/birds into the herd/flock... that was how I got Marek's, so I have learned my lesson, but have raised foals and chicks from my herd and flock within the flock. My land has been grazed by horses/donkey for the past 50 years so plenty of opportunity to build up a worm load and no lime applied. My chickens are currently penned in a coop and run that has had chickens in it for 10 years or more, and I do not poo pick the run, it is currently deep litter and as I said, one young bird in that coop with a compromised immune system did develop a worm overload, so it is not like there are no worm eggs present, they are just not causing major infestations in healthy chickens.
These things all lead me to believe that heavy worm burdens are not common in healthy stress free animals and that regular worming is not essential in my situation and seeing the odd worm in poop every once in a blue moon, is not something to panic over and "Nuke it from orbit" as the poll option is worded. I would point out that the OP is in a very similar climate to myself, which may be relevant. Their birds have however come from a rescue where they have no doubt mixed with lots of other birds from different sources, so there may be an opportunity for infestation there due to stress and exposure. I imagine these rescue hens have come from commercial factories where they have never eaten off the ground and may have compromised immune systems due to lack of exposure, so worming in this case may be relevant.
I just don't think it is a viable expectation to keep chickens worm free and it may even not be healthy to try to do so. As I said, the immune system of the bird should prevent that but if it never gets the chance to build up that response because chemicals are being pumped into it regularly, there is the chance that if a treatment is missed for some reason, a worm burden may build up.
Worms, like anything, have a finite life span and when they die they will come out in the poop, most likely entire since they live in the gut beyond the gizzard where food is ground up. Seeing the odd one is therefore not a huge issue in my book from my experience.