To be totally honest, I wouldn't pick one seat for the things you want to do. I'd get a seat to cover the rear facing stage, bonus if it has a forward facing mode too, and then a high back booster or possibly a combination high back booster/5 point harness seat. There is a Graco R129 high back booster which is less than £50 which is fine, and you can get rear facing/forward facing seats by Joie or Graco around the £100 mark or less.
I don't like the MyBabiie seat - it has terrible reviews (sort by negative, the positive ones are always boosted by bots/influencers), the instructions are absolute gobbledegook (more so than car seat instructions usually are - these are verging on dangerous!) and it never seems to fit any child well at any stage. Top tether is also very annoying to combine with spin seats. All the youtube reviews I've seen of it and even their own promo videos it's not used correctly, they either don't bother to attach the top tether or something else is wrong.
Joie Stages is still available at Halfords, and would be fine, but won't fit the 6yo for very long, so you're likely to need a high back booster at some point anyway unless you are happy to keep a backless booster around for older children. You could look at Joie Every Stage R129 (or Graco Slimfit R129, they are the same seat) which are very similar to Joie Stages, but the high back booster mode lasts longer. If you are 100% committed to having just one seat, this is the one I'd recommend. The safety rating on it is surprisingly good (it was ADAC tested last year) and the price is nice too - it's about £150.
The only downside of this is that it doesn't have a forward facing mode with the harness - it's either rear facing, or it's a high back booster. No in between. So if your young relatives end up forward facing in their parents' cars aged 2 or so, they might not be totally happy about RF in yours (though, you might be able to sell it as a "cool fun backwards seat!" - we used to love travelling in the boot seats of my dad's Land Rover, because they were backwards.)
If your car is older than 2013, then you might not have top tether anchors, so might not be able to use a top tether seat. In cars produced from 2006-2013 they had them about 50-60% of the time. If you are unsure you could check the car manual as it should say in here. You can't retrofit top tether anchors, or use other similar points like luggage anchors. If your car is newer than 2013, then you will have top tether points and they should be clearly labelled, but they can sometimes be stiff or rusted if they are a D-ring type that have never been used.
The storage boxes as you describe are a pain, they are totally notorious for child seat legs not being suitable. You can check your car manual as sometimes they have guidance for what to do. Some child seats have an extra long leg, which is allowed to be extended to the bottom of the box. I can't remember which offhand, unfortunately, but you could certainly take the car to John Lewis or similar to try. If this is not allowed in your car, then you can usually buy a foam filler from the car manufacturer which will allow you to put a support leg on top of the lid, but at who knows what cost and hassle to find one. For whatever reason sometimes car manufacturers use the term "stay" to refer to the leg. Otherwise they say support leg/support foot.