Gavin,
You don’t mention which flavour of Swallow you have but I’m guessing from the size of your Garmin 723xsv, combined with your intention to go trailer-sailing that you’ve upgraded to a BC23 rather than a BC26. If so, then you probably don’t intend to stray outside of coastal waters very often.
Did the Garmin not come with charts already installed, as in the good old days? My elderly 557xs included UK & Ireland BlueChart G3 charts. Or have they and their subsidiary Navionics found another way to gouge their chartplotter customers? Rhetorical questions! And here’s another one - am I alone in finding the Garmin website, with its multiplicity of plotter and transducer models, utterly confusing?
Back to the matter in hand, the choice between UK & Ireland Lakes and Rivers - Inland Marine Charts and the UK South Coast version looks like a no-brainer. The latter’s coverage appears to be a substantially chopped down version of the former, plus the Scillies and a bit of northern France, with no difference in price. You then need to make a choice between Navionics+ and the more expensive Vision+ version. The extra cost gives you garish relief shading (see below), better satellite imagery of land-based features, 3D imagery and some aerial photos. The price of both versions includes a one year’s subscription to updates through the ActiveCaptain app, after which you have to renew if you want to continue to update. More oligopolistic gouging. For the initial purchase, go for the download version rather than the SD card. If you lose or damage the card, you’re stuffed.
My 557xs is not compatible with these new charts and is getting a bit long in the tooth, so I am facing some decisions. I’ll be posting on this in a month or two but the clue as to what comes next is in this thread
https://www.swallowyachtsassociation.org/smf/index.php/topic,2005.0.html.