Clean/good seaside around london?

Nearest seaside is Brighton, due south of London, but the beach isn't sandy.  For that you need to head to Camber Sands, East Sussex. There is a sandy beach Lido at Ruislip, but swimming is not technically allowed. You still need to be aware of Coronavirus and not all facilities are open.

good luck- London is on a River not the Sea

You have the Kent coast like Margate Ramsgate, Broadstairs and Dymchurch and then  Camber  sands just over the Sussex border. 
Further along the coast Hastings and Brighton are stony before more nice beaches at Bournemouth. 
Going east into Essex, Clayton is probably the best bet.
All of the above are 80-100 miles from London though .
Southend is the easiest seaside town to get to from London but has only a thin strip of sand before you get to mud. Nice.

Forget all of that just now. All beaches are full of idiots ignoring corona virus rules and social distances . Complete morons.

London is on the Thames River, not near the sea. You'll have to travel. I am from the USA, but once was in St. Ives, at the southern tip of England. It was a lovely little village for tourists and artists, but also known for surfing. Long sandy strand, beautiful waves, check it out.

How close is 'around'? There aren't any sandy sea beaches in London itself.....the Thames makes sure the water's edge is mostly mud until you get some distance downstream from London. Within London, there's freshwater swimming and sand at Ruislip Lido.

How much seaweed there is at any place at any given time is unpredictable. Might be lots today and none tomorrow, depending on weather.

Why don't you google 'sandy beaches near london uk', which will bring you a number of websites with lists of beaches not far from:London, with photos and descriptions. You could also try 'sandy beach essex coast'.

You have the Kent coast like Margate Ramsgate, Broadstairs and Dymchurch and then Camber sands just over the Sussex border.

Further along the coast Hastings and Brighton are stony before more nice beaches at Bournemouth.

Going east into Essex, Clayton is probably the best bet.

All of the above are 80-100 miles from London though .

Southend is the easiest seaside town to get to from London but has only a thin strip of sand before you get to mud. Nice.

You do know that London is inland, right?  Google maps is your friend. Go to maps.google.com and put in "london". Then scan out until you find a beach. BUT- England is in the north Atlantic Ocean- where the water is cold and seaweed abounds year round. I think you want the Spanish beaches on the Med. 

When the tide goes out (Thames),  a sort of beech emerges on the south bank.  Get down there before the sand sculptors and random mudlarks.  Not sure that it’s a good idea to go swimming though.  Bit hazardous in more ways than one.

Brighton is the best....very clean but can be crowdy.Also plenty to do if it rains.

Frinton on sea. One hour + by train from London