Sorry, I skim read and missed the part which said you had some tyres already.
Have a look on ebay for the wheels, if you search "a6 winter wheels" there will be adverts pop up from German sellers that will ship to the uk. It's law to fit winters in Germany so there are lots in Germany. I've seen quite a few o. e wheels recently. There's an example below. There are a few styles, most will have tyres so it will be a case of looking for ones with not much tread that will be cheaper. You will find a UK seller but there are just a lot more available in Germany so if you them quickly Tyra will be your best bet.
If you want new, then audi will sell you some, they had them in the showroom when I had my car serviced a couple of weeks ago.
https://www.ebay.co.uk/itm/194485511286?fits=Car+Make%3AAudi|Model%3AA6+Allroad&hash=item2d483d6476:g:YMAAAOSwq3RhgSdw&amdata=enc%3AAQAGAAACYPYe5NmHp%2B2JMhMi7yxGiTJkPrKr5t53CooMSQt2orsSd3M8ngECaRA0SmyU4KT%2FnomKR%2Bekfv00jipkJOU6R0H0IHwdw9uEIHN8zgy9CTJ0pADp0H9n17gAxhbVKBrfv6U%2FXJEwQMdsyIp86uJLQIfIhFh9iUxW%2BE%2Bhz8m5EY%2BvpqTtWdw3LYgHlAzFMxEKQWm5AO68UUdBjDA%2FT8MXNHc5pCFvmgdCEx1FHJ5bYO6yN4X4yegz1Uj0jJp85jtA%2BzpcMrALWOftgN7utid0dtOt4ST%2BSwYDzh45dDDtla1YDKiG%2FicklpxQlcZdYXCOtqmW06Dm9AgvXpl7a3RdGqZ88DUmKUT3aY2sOcKKbi1YJAL1wT%2Bvm9x8%2FYMN1Kg%2BXJvWq70wjMAdZw5Y5gx5G5W6EUc04MWgJvVKHslXUL16tN%2B0ttAA5EVvurIYDOwzP1d6pkKrh5RZ1rVhRF2aXbrLkfN5UzrNAFcAF8tpi%2BfXJ7P8gLDqe4pDuF1HhELo3U1DFdnjKiRWHaDwkK42CXv1y6EYWZm3wZWvPsJXy6yihDB7SWwsA1WRcuqmiZwhqwUk7nNpYVe8YX7ubtOSZRQyJZ14tncRPzPA2CcKO78tdmEdd3vkK0WDqYg8nJxK%2FJS6G5SQdEJcDWSovbWAjcdv21ERz01QoK6xFylyrbAAt6mEt%2F8X7Dii24uWHiuP0qIvcCzJEE0ihy1QIjpbZbO7fgeN2NVmAY45OliC4IurYkIheefU%2F52ZdGYjvF%2FdaWzaVbl12kY2mtFG3ONm5kDgubhFfygILN5saPhjx6kN|clp%3A2334524|tkp%3ABk9SR6r3y4uhXw
Edit:
I've just noticed you said steel wheels, 🙄, I think I need a coffee to wake up ☕🤣
There are lots of 17" alloy wheels on ebay in the uk with summer tyres on, might be worth looking at those and swapping tyres as they aren't expensive.