I will consider any part of the UK. It would be me and dh, 3 young kids (boys) and my in-laws. But we have no-car (and don't want to rent one) so would need to get there by train (from London) and then be able to walk to shops/village maybe a beach etc?
Milford-on-Sea - train to Brockenhurst or Lymington, then bus. We have beaches, restaurants (Italian, Thai, Indian, creperie), three pubs, proper food shops etc. Buses from here to Bournemouth, Southampton and even a weeny ferry to the Isle of Wight! But please don't tell anyone!
If you don't mind a longish train journey, how about Devon or Cornwall?
Penzance is great, with loads to do in that area. We did it years ago by train, with 3 under-fives. There is masses to do in the town itself, including pebbley beach. If you're into walking, you can walk to villages like Newlyn or Mousehouse. There are buses to places like Marazion, where you can visit St Michael's Mount. Get a train to St Ives for sandy beaches.
Or Newquay, again we did this by train one year. Not really my cup of tea as a place, but has loads of good beaches and things to do - swimming pool, zoo etc. Don't know how good buses are for venturing further afield.
Have a look at a rail map to see what resorts are on the train line - off the top of my head, in Devon you have places like Dawlish and Paignton, don't know how easy a journey it is from London.
Or venture 'up north'. You can get to Whitby by train, although it would be a fair journey from London. You can spend a whole week in Whitby and entertain the whole family without leaving the town. Or go on the steam train from Whitby to Pickering or one of the other places on that line.
What sort of cottage are you after? How many bedrooms etc? I might be able to suggest a cottage from one of the places we've been to.
Sheringham in Norfolk. 1 hr 50 mins to Norwich from Liverpool Street, then another 50 mins to Sheringham. Lovely old-fashioned seaside town. Lots of independent shops, great beach, tropical leisure pool, some nice restaurants. The train goes right into the town, so not much walking required. From there, a steam train runs along the coast, and also rail and bus links to surrounding villages and towns. Highly recommended!
Thanks everyone. Devon is out as my dh used to live there. Sheringham sounds nice - I will file that aaway for the future.
In the end we have booked Weymouth. It sounds fab with loads to do within walking distance (sandy beach, country parks, redeveloped harbour area, sealife centre etc) with a lovely cottage on the sea front.