arturas , when you can fish depends on where you are in the UK. In England and Wales the regions may have slightly different dates when you can fish for Brown Trout/ Sea Trout/ Salmon. Also remember that to fish for either Salmon or Sea Trout on rivers in England and Wales you must have an EA Migratory licence.
Have a look at these pages from the EA website for information:
Regional byelaws incl dates
Environment Agency - Rod Fishing Byelaws
so if you are in the NW of England, you can't fish for sea trout between 01-Oct and 31-Mar; you can't fish for Salmon between 01-Nov and 31-Jan;
but in the Midlands, you can't fish for sea trout between 08-Oct and 17-Mar; you can't fish for Salmon between 08-Oct and 31-Jan. We don't like to make things simple
National byelaws for Salmon and sea trout
Environment Agency - National salmon and sea trout byelaws
Andrew