'local' legislation is already in place on at least one river - maybe on others. The river I fish has one very restrictive rule in place; mandatory catch and release for half of the season, including fish that die on return to the water - if it dies, you let it float away. If you're found taking it home, or the bailiff finds it in your bassbag at the riverside, you'll be prosecuted.
I think the reasoning behind it is this is a last-ditch attempt to stop people taking fish away indiscriminately, and saying that 'sadly, it died when I tried to return it', when in fact it didn't - it was actually a blow on the head with a priest that caused it's demise. This river has seen such a massive decline in salmon numbers that I'm sure it's felt that rules like this are the only way forward if the salmon is to survive there.
Whilst I think that this particular rule is somewhat controversial, (if they die when attempting to return them, they should at least be recovered and handed in to the EA, perhaps) I am still prepared to let all my fish go because I stick to the rules, and I want to see the numbers of salmon increase as time goes forward. One taken here, one taken there, has a very detrimental impact on a river such as this. BTW, the EA, the local Angling Association and the riparian owners are also actively supporting the river with a restocking programme. The estuary nets were also bought out, and that's got to be good for the river.
If the river you fish has a good, sustainable head of salmon, then one for the pot is fine. Sadly, as I said previously, many UK rivers are definitely not in this position.
If you really must take a salmon home, then go and fish a river with sufficient stocks to allow it.