I don't live in the UK.
Some of the chicken breast fillets for sale here contain a "light salting marinade", which is shown in the small print. Of course this is just a way to sell you water rather than chicken.
So the first thing is to closely check the ingredient label of the stuff you are buying.
Also, if you are buying cheap stuff, at the bottom end of the price scale, then it's quite possible the water has been added (either through injection or tumbling in a large machine), and they just haven't declared it to the customer. It is cheap for a reason.
With 8 years experience in the chicken industry, here is my assessment of the market:
Expensive - Fresh chilled fillets in retail packaging, from higher end supermarkets. This meat will be from an established supply chain with partner suppliers. Quality checks and auditing are at an excellent level. Their traceability is so good, that from the code on the retail pack, they can trace back to the actual shed on a particular farm, where the bird was reared. And tell you what it ate and when, how many hours of sunlight the birds were given per day, etc, etc.
Mid price - Cheaper supermarkets fresh and frozen. Still traceable to supplier, but possible that "cost reduction" has been applied somewhere.
Cheap - Raw material will likely have been spot-purchased on international markets. The buyer picks up his phone and calls contacts, asking, "what do you have available at what price?". Traceability non-existent. You have little idea what you are buying.