Getting to the sea from Oxford

I may be biased because I grew up there, but I'm a big fan of the beaches around Christchurch, Bournemouth and Poole. There is 7 miles of continuous sand from Sandbanks (Poole), past Bournemouth and on to Hengistbury Head (Christchurch). From Sandbanks, head across the mouth of Poole Harbour on the chain ferry, then you've another 5 miles of sandy beach along Studland Bay.

Cross Country Trains run one direct train per hour from Oxford to Bournemouth, and the journey takes just under 2 hours. (Currently it's the xx:16 train). You can also get there by changing at Reading and Baisingstoke, which sometimes makes sense if you've just missed the direct train!

If you're aiming for the beaches at Bournemouth, it's a 10-15 minute walk from the station to the nearest patch of sand (Holdenhurst Road / Meyrick Road / down the steps). For the Pier and the Gardens, there are plenty of buses from the station.

For Sandbanks and Studland, change at Bournemouth for a train to Poole (it's normally a few minutes behind on the same platform), then walk through the shopping centre to the bus station. Sit on the top deck if you can, and enjoy some stunning harbour views as you head to the beach!

Otherwise, in a car head down the A34, then M3, M27, and get out a book. On a nice evening, especially a Friday night, you then have a 20 mile carpark across the New Forest... When you finally make it along the A31, take the Spur Road to Bournemouth or carry on for Poole. On a good day it's about 90 minutes without stopping, on a bad day 4 hours is not unheard of!

Getting to Hayling Island is fairly straightforward, a 1h 40m drive. dayoutwiththekids.co.uk has some other suggestions. Finding public transport links shouldn't be too difficult.

Take the Oxford tube to victoria station. You can get to Several beaches from victoria. For example

Brighton

Brighton is a Shingle beach Trains run direct from Victoria to Brighton

Margate

Margate is a sandy Beach Trains run direct from victoria to margate

The BBC have a page listing good beaches from around the UK and how to get to them

http://news.bbc.co.uk/1/hi/uk/2960774.stm

The other option is to go west to North Somerset or near Bristol. Weston-Super-Mare is renowned as a resort (although sometimes ridiculed for its mud-like sands); there are some others near Bristol such as Portishead, which are closer but not as big. Severn Beach is even further up the Severn estuary towards the east, but is less well served by trains, and I'm not sure how nice it is.

Weston can be reached by train in about 2h15m-2h30m, although you have to change at Didcot and Bristol. You can also drive, down the M4 and M5 most of the way; Google Maps says just under 2 hours. If you've a car you could stop before then, e.g. in Clevedon which is further east on the north Somerset coast; it is pebbly but also a good option for traditional seaside.

This works pretty well for me

https://www.distantias.com/nearest-beach-to-oxford-great_britain.htm

From Oxford, you can pretty much jump on a train and get to most places but getting to the coast is seldom direct.

Oxford to London and then choose either East, South East or South beaches via train. In short, not quick.

Driving is probably the best by far

It depends on your definition of a beach, but I really like pool and surroundings at Rushey Lock and Weir Visit Thames Google Maps. This is about 20 Km from Oxford.

The grassy area to the south of the weir pool is nice for sitting and picnicking. There is a very small (2 or 3 yards by 2 or 3 foot) sandy "beach" at the south-east of the pool which children may like to play on and provides easy access to the water. The pool is deep, and is very pleasant to swim in. The weir has a concrete outflow that ensures that a stopper does not form, meaning that one cannot get trapped by the current. If the flow rate of the river is right it acts like an infinity pool, one can swim at a constant rate while staying in the same spot. I believe the water quality to be very good. It is upstream of the large population centres on the Thames, and there are shells of swan mussels that I think are a bioindicator of good water quality.

It is accessed from Buckland Road at Tadpole Bridge. There is a car park for anglers on the west of the road, and The Trout Pub on the east. From the bridge it is 1.2 Km, you can walk along the footpath along the bank or swim up the river. If you are not driving the S6 gold bus from Oxford stops at the A420 / Buckland Road junction, from there it would be about a 3.5 Km walk. I have not been there for 2 or 3 years now, and have never taken public transport, so this may be slightly out of date.