Grid ref posts on the South Oxford Canal, two flypasts and a butterfly

Every so often, on the South Oxford Canal, we came across white posts set in a short length of post-and-rail fence.

These white posts have an Ordnance Survey grid reference stencilled in red on them.

Some of the grid ref posts are obscured to a greater or lesser extent by foliage ...

... and one was covered by a hoodie.

Some of the fences have a two letter and two digit code. This one is OX55.

Here are two more I found, all snapped from the moving boat so not the best quality.

Does anyone know what they are for? I checked with a GPS once or twice and the grid references seem to be accurate.

We moved only three miles today, from Fenny Compton to the north side of Wormleighton Hill. We tied up with a great view over farmland to distant Warwickshire hills and had a barbecue. There was a sound of aeroplanes ... and it looked like it could have been a Battle of Britain Memorial Flight going over.

A few minutes later four more planes flew over in a diamond formation.

I don't know what these were - come to that, I don't know what the first flypast consisted of - so can anyone tell me please?

And a final identification of a flying thing if you could: this butterfly on brambles.

Actually, a look on the internet shows me that this could be a meadow brown.