Well, if you've ever tried walking in countries like Spain or Italy, we should celebrate the fact that we are spoiled for choice. You can always get by with an OS, so why would you ever choose Harvey?
First, you should understand that they offer two main series - the SuperWalker maps at 1:25k and mainly centered on popular mountains, and the newer BMC ...
A path indication on the map means that when the mapper mapped it there was physically a path on the ground. It gives no indication as to who is or is not allowed to use that path.
The brown background indicates "access land". You can walk on this land subject to some restrictions. This applies regardless of whether there happens to be a path. "access land" ...
According to the Ordnance Survey legend, those are
Civil Parish (CP); England
or Community (C); Wales
See page 6 of the legend under boundaries. There are no other small black dots on the legend.
I've used OS 1:25k and 1:50k extensively in the UK. I also have a little experience of using 1:40k Harveys maps (Lake district, for mountain marathons).
I have been impressed with the Harveys: in detail as well as scale they sit nicely in between the two OS scales. So you get very detailed contours (nearly as much as the OS 1:25k), but less of the clutter ...
On Ordnance Survey maps in England and Wales, the brown background shows access land: see this legend. On such access land, access on foot is permitted anywhere. You cannot cycle here.
A path is a geographic feature. A public right of way is just that, a right. It may be on a road, a track, a path, or simply through a field. If you look carefully, you ...
I know several people who recommend the Harvey maps, so they're obviously not terrible.
The main advantage is the 1:40,000 scale which offers most of the detail of a 1:25,000 map while covering a larger area. Additionally, they don't have many irrelevant features like county boundaries and slightly more colourful style than OS maps, which make it easier to ...
The mauve symbol is new, and represents "Natural Resources Wales".
This appears to be Open Access Land. It indicates the type of land bounded by a mauve outline (a fragment can be seen in your image).
It can be found on this Ordnance Survey symbols PDF.
Ordnance survey were asked on twitter, and replied that:
"Hi thankyou for bringing this to our attention. The circles are actually blemishes on the data which are being removed and will no longer show in future updates. Sorry if this has caused any inconvenience."
Of course they might say that if it were a fictitious entry as suggested by Darren's ...
I'm not sure what app you're using, but they're from the app, and not part of the base map.
You can view the OS map in Bing (yes, it is useful for something!) here and the circles (octagons?) don't appear. What made me think they're not on the base map though is the line weight. The screenshot embedded kin the question has been through a bit of rescaling, ...
Nothing appears there on the satellite view, although the tree cover could well be concealing whatever it is.
My best guess at the moment is that this is a fictitious entry that is added by cartographers to catch out copyright violators who blatantly rip off their work.
These fake entries also go by the names “Mountweazel”, “Trap Street”, and others ...
I believe that what you are looking for is,
Gavin Brock's Ordnance Survey Map KML Overlays for the UK
Same one found here.
The Charles Close Society for the Study of Ordnance Survey Maps
Any of these should put an Ordnace Survey overlay into Google Earth and then you could combine that with the .kml file you already have.
The closest match with the 1:25000 OS map symbols that I can see is #16 scree.
The circles are not shown on the 1:50000 map and there is no scree symbol for that map range either.
The circles are all the same size: perhaps there are just a few small distinct patches of scree.
I had a look through the symbols/legend for the ordnance survey maps here (PDF), and could find nothing that matches exactly the type of symbol you have shown.
The closest were boulders (under Heights and Other Natural Features), but these would be normally oval/irregular shapes rather than circles, and with a heavier weighting on the line. The other ...
You can convert to GPX eith with Chris h's suggestion above or http://gpx2kml.com/ will convert Kml to gpx and back.
I really like Viewranger, is a great phone app that will allow you to have GPS navigation with your route overlayed onto an OS map. I'm guessing that you're after a more desktop approach though.