Today, Monday, is a Bank Holiday aka public holiday. The first Monday in May is a spring break for schools and some shops. As the name suggests, banks will be closed. It is a long weekend.
If you are here in London, you won’t notice too much that’s different. Shops will be open probably, perhaps with Sunday opening hours but probably not even that.
Trains, buses etc will still run. Restaurants will be open.
Depending on the weather, places may be more busy so you are advised to book in advance if you can.