The former change will already effectively make the "stay at home rule" die. That said, during parts of last year in Wales and Scotland, non-essential shops were open but the "stay at home rule" still existed... with defined reasonable excuses to visit such shops!I expect the only changes to the legislation on 8 March will be “open air recreation” being added back into the exceptions (from which it should frankly never have been removed - it wasn’t for the November lockdown; a cynic would suggest it was removed purely so they could give us something meaningless back!).
Then at the end of March the legislation will change from “no person may be leave from the place where they are living” to “no person may stay overnight away from the place where they are living”. That is how it worked last summer.
When the second of those happens it’ll become much harder for the government to stop people leaving the country unless there’s some further change to the legislation to prohibit it.
In truth the biggest barrier to international travel, both now and in the medium to long term, is the prohibitive cost of testing - you need to get tested 6 times for a return trip to lots of European countries at the moment, and that's so you can cut it down to 5 days' quarantine on either side. Also the risk of a country being declared a red list country at a moment's notice. Tests are around £80-100 a pop, so that makes a quick getaway totally unaffordable for most.