As far as the chemicals are concerned, I remember a Question Time when an audience member who was involved in importing them, said that post Brexit this would be a problem. A Brexiter on the panel, I can't remember who, rudely put her down.
I remember the episode as well - it was Jacob Rees-Mogg - but the chemicals were imported from non-EU countries (China, India etc.) and then exported to the EU.
Non-anecdotally, the fact that Nando's and MacDonald's don't have any problems on the island of Ireland is pretty telling as to where the real issue is.
Although KFC Ireland/IE had some problems in August;
www.spin1038.com/news-and-sport/kfc-warns-of-supply-issues-1238173
Nando's is interesting - probably a combination of factors:
Northern Ireland surely didn't have problems because it has a poultry population of 25 million birds (and a poo problem!) - and only 7 branches of Nando's;
www.theguardian.com/environment/2021/jun/23/poo-overload-northern-ireland-could-be-forced-to-export-a-third-of-its-animal-waste
There are 12 branches of Nando's in Ireland and 430 branches across England, Scotland, Wales - 40/50 branches were closed for about week.
Reports that Nando's were having supply problems surfaced on the 15th August. Looking at their website they had a 'free' 1/4 chicken promotion for 2021 GCSE and A Level students collecting results - and also for 2020 students because they 'missed out' last year. So how popular was the promotion?
www.nandos.co.uk/resultsdaydash
www.manchestereveningnews.co.uk/whats-on/food-drink-news/free-food-deals-teenagers-getting-21292472
Avara, one of Nando's suppliers (quoted in an article up thread/difficulties attracting new staff) are featured here as well;
23 June 2021 -
Previously operating five days a week, the Dudley plant is now running seven days a week, following a huge increase in consumer demand.
Production records have been smashed, with the factory producing hundreds of thousands more units of product than in 2020.
Production capacity has increased by 30%...
www.foodmanufacture.co.uk/Article/2021/06/23/Avara-
Foods-poultry-factory-adds-130-jobs
Nando's doesn't source it's chicken from Poland but avian flu might have had a knock-on effect;
April 2021-
One of the UK’s leading poultry suppliers has warned of a chicken shortage over the coming months after an outbreak of avian flu in Europe.
Birtwistles, which supplies butchered poultry to thousands of hotels and restaurants across the country, said stocks of some cuts, such as small breast fillets used in chicken burgers, are down by 40 per cent.
In a letter to customers, seen by i, the managing director, Glenn Eastwood, said it was “urgent and necessary” to alert businesses to potential shortages of poultry from Europe into the UK, which could last up to 12 weeks.
The letter said that Poland, the largest supplier of European Poultry into the UK, has had an outbreak of avian flu that resulted in the slaughter of more than 20 million chickens last week to reduce the spread.
inews.co.uk/news/consumer/chicken-shortage-looms-after-avian-flu-outbreak-in-europe-warns-poultry-supplier-970156
I haven't even mentioned the 'pingdemic' or the HGV driver shortage yet...