OK so the Britax you're looking at is expensive because it's got a modular base and a spin function, both of those things are trendy at the moment but not really necessary IMO (Certainly not worth £480)
Britax are a great brand, but actually any infant carrier is good for safety, this one doesn't get significantly better values than Graco Snugessentials (for example) - Graco not being noted for getting especially good crash test results (they are usually average). This is probably because infant carriers are small and rear facing and one piece shell - so they tend to do really well whatever the brand.
You're looking at the iSense, which is the one that has lights built in. The BabySafe 3 i-size is exactly the same seat without the lights. It's the Babysafe 3 and i-Sense that have the new connectors which go on the Maxi Cosi type adapters (that most pushchairs are compatible with). Those are the two new Britax baby seats released this year, their older models have the Click & Go (Britax specific) adapters instead which don't go with as many prams.
You mentioned the internal recline - this is not really a recline as such, it's more of a flattening of the angle. If you look at the product page and scroll down to "Features" it's illustrated in "Lie-flat technology" - basically, when you have the straps on one of the lower settings, a flap comes into the car seat and lifts the baby's bum up.
www.britax-romer.co.uk/car-seats/baby/baby-safe-isense-bundle/3023.html
If this feature is what you're looking at mainly in the Britax, you could look at an older Britax model. Baby-Safe plus SHR II, Baby-safe i-size, Baby-safe 2 i-size all have this feature. The i-size and i-size 2 are also compatible with their own flex base (which I'll get to later!) which is cheaper than the i-sense flex base. The main issue with these older models is they use the old Britax-specific pushchair adapters, although you can still get those for a lot of pushchairs, especially if it's popular in the US or Scandanavia.
Other car seats do a similar thing with the use of inserts instead, for example Maxi Cosi infant seats have a wedge insert that lifts the baby's bum up in a very similar way, which works particularly well. Pebble Pro is my favourite, but Pebble 360 is more lightweight and Pebble Plus is an older model that is sometimes cheaper. Cabriofix i-size (same as the old Tinca) is cheaper as well. NB you should not add third party inserts to car seats, so it's best to choose one with inserts that you like.
Avionaut Pixel is well worth a look, this is a new brand in the UK, Polish brand, they have a very unusual insert in their infant seats. Most infant seats, the baby's bum stays in the same place for the use of the seat and the head support/shoulder belts move up as they grow. With this seat, the shoulders stay in the same place as they grow and you start off with almost a tiny booster seat to boost their bum up to meet the shoulder straps, with parts which are removed as they grow and this keeps them in a flat position. Unfortunately they have removed the very useful diagram from their website, but Venicci who also use their branded seats have got one on theirs:
www.venicci.co.uk/product/venicci-ultralite-carseat-black/
That insert is in the Venicci Ultralite, Avionaut Pixel, Pixel Pro, and Avionaut Cosmo seats at various price points. All of these seats are extremely lightweight.
There are other brands which have wedge inserts to flatten the angle for newborns as well BTW - I just don't know how good they are - you can get a sense literally by looking at them in shops. The Nuna one and the Joie ones (except the i-Level) seem to not be great at flattening the curve based on what people have said online. I have a Cybex and it's OK (the Maxi Cosi and Britax are better).
For car seats that actually recline in the car, Joie i-Level is the most economical. It's heavy, but it has an actual hinged recline and it comes with the base which makes it incredibly good value if you want a seat with a base - it's one of the cheapest seat + base combos on the market, even without taking the recline feature into account. (John Lewis seem to have it for under £200). The downside is that you can't use this seat without the base, it doesn't seatbelt fit.
The flex base with the Britax is not quite as flexible as it looks in that sales video. These bases are essentially designed to counteract a slope in your vehicle seat and aren't intended to provide extra recline, it's probably not recommended to use it that way. When you use an isofix base with angle adjustment, what you'll find is some kind of sensor inside the seat which measures the angle for you and helps you find the correct angle for the base by indicating green. You can then generally only use it on the green position. Over-reclining the seat would cause an issue because the seat is not designed or crash tested at that angle. This is a better demonstration of how the angle adjustment feature works (and you can see that even though that seat looks very angled, they didn't end up using one of the steeper positions to correct it.)
There are other flex base options; Graco have a flex base in their Snugessentials range, which is very cheap (Graco seats don't use the Maxi Cosi connectors; they only go on Graco prams) Joie have a flex base which fits most of their seats (not the i-Level), Cybex Aton and Aton 5 bases have a less technical function which is hinged isofix allowing insertion of a towel just below the isofix points to raise it up. And the low-tech solution to sloped seats is just to take the seat off the base and belt fit it, which allows you to tip the seat back to counteract the slope, that works with basically any brand unless it has a very flat bottom of the seat.
In general, I think new car seat + second hand buggy is a great idea anyway, but you absolutely don't need to spend £480. You could get a seat and base combo for about £200-250 or a seat by itself for under £100 or up to about £150, and it will do everything you want it to do.