I've been to a decent amount of it, I'm 24 and I have probably seen more of it than most people under 25 so that's not bad.
In terms of the Far North, which is where I am from, I have seen pretty much everything several times over. Newcastle, Durham, Sunderland, Middlesbrough, Hartlepool, Carlisle, Hadrian's Wall, Bamburgh, Cheviot Hills, Lindisfarne, you name it.
I mean, those last 5 I listed are all in Northumberland, which is probably one of the most beautiful cultural regions of England, perhaps all of Europe.
Seen most of the Dales - really love the town of Richmond down there, and Gordale Scar is outstanding - and pretty much all of the Lake District - done a few of the mountains there Helvellyn, Scafell Pike, Blencathra - to the point that I have seen pretty much every town there several times over, I used to go to Ullswater a ton as a kid.
In the rest of the 'North' if you want to call it that, I've been to Beverley, Blackburn, Blackpool, Burnley, Golborne, Harrogate, Helmsley, Hull, Kendal, Leeds - my dad is from here - Liverpool, Manchester, Northallerton, Penrith, Preston, Ripon, Rochdale, Sedbergh, Sheffield, Skipton, Warrington, Wakefield, Wetherby, Whitby, Wigan, York, probably a few more, in terms of cities, plus not to mention a couple rural places around those...
In Cheshire, I've just visited Chester properly, but would like to do more of that county. In Derbyshire, I have visited Edale in the Peak District, plus Winnats Pass and absolutely loved it down there, climbed Mam Tor as well. Been through the edge of Derby.
In Staffordshire, I've been to Lichfield - which has to be one of the most underrated Cathedral cities in Britain - plus Stafford, been through Stoke-on-Trent, plus a few places in the Staffordshire Moorlands, including the Roaches and Biddulph.
I went to Shropshire, I loved Shrewsbury, been to Church Stretton and Brown Clee Hill down there too, plus Ironbridge.
In Lincolnshire, I've been to Grantham - for the statue of Margaret Thatcher - and Lincoln itself.
Never been to Birmingham. I love Stratford-upon-Avon, in Warwickshire. In terms of Gloucestershire, I've been to Painswick, Cirencester, Bourton-on-the-Water and Cheltenham.
Went to Cornwall and Devon as a kid.
I only properly visited East Anglia recently, went to Camrbridge and Ely in Cambridgeshire, and King's Lynn in Norfolk.
I've been to a bit of London, but honestly not a whole lot of it. I've got family in Woodford Green, used to go there at least once a year from basically when I was born to like when I was 13, plus went again when I was staying overnight for gigs when I was 19, 20-ish. I went to the Houses of Parliament when I was 16 to watch a debate. Also done a bit of Kensington... can't decide whether London is overrated or whether I should explore it further.
Been to the port at Dover a few times, other than that not seen a whole lot of the South East apart from the airports.
In Wales, I really love Snowdonia/Eryri, climbed Snowdon/Yr Wyddfa, plus I've stayed at Llyn Gwynant, and visited Beddgelert, Llanberis, Llanrwst, Betws-y-Coed, Porthmerion ... been to Baumaris, Holyhead and Llanfairpwllgwyn(...)goch on Anglesey. Been to Bangor, Llandudno, Gwrych, in North Wales too.
In South Wales, I've only ever been to Cardiff, but I did like it.
In terms of Scotland, I've been to Edinburgh a million times, plus Loch Ness, Fort William, Glen Coe, Lochgiliphead, Loch Lomond, ... Lochmaben, Jedburgh, Hawick, you get the picture.