Welcome to the forum
A few I would recommend in the £400 price tier;-
Orient have the Ray, Mako, Kanno and Kamasu to name a few in this price tier (they also have things like the Bambino which are well regarded), I have a Kamasu and at its price point there's not a lot better than it on paper.
Seiko also have some offerings here in their 5 Sports range, for these I would advise avoiding eBay as there are a lot of rebuilds and franken-watches which are not worth it.
For Orient and Seiko the likes of
https://www.amazon.co.uk are a safe place to shop,
https://www.cwsellors.co.uk stock Seiko and if you are happy to wait
https://www.jomashop.com is also an option that stock both (you might have additional import related duties to pay with JomaShop but on my last Bulova purchase I didn't).
If you want something that's a little more colourful and different maybe check out Phoibus or Zelos, they've got some great offerings and you can just buy direct from their
websites:-
Phoibus:- https://phoiboswatcheurope.com
Zelos:- https://zeloswatches.com
£400 will also put you into Hamilton Khaki territory so I'd definitely recommend checking them out as they are a great watch and well regarded in the community.
https://www.hamiltonwatch.com
A couple of brands that I like which don't get as much love are Junkers and Iron Annie, they have a more Bauhaus inspired take on design and produce some fantastic looking minimalist pieces.
https://shop.junkers.de/en/
https://www.pointtec.de/ironannie/en/
An outsider is Islander Watches, these seem to be great value pieces and the range is only growing and growing as they develop their own brand (again, these are in the US so there's a chance you could have additional import duties to pay when the watch lands in the UK, but since Brexit you'll have this with everything that's not already in the UK). Long Island Watch also stock Seiko and Orient so it's another good place to check for them.
https://www.longislandwatch.com/Islander_Watches_s/2006.htm
Beyond that brands like Bulova, Casio, Citizen are all worth checking out as you never know what might catch your eye.
https://uk.bulova.com
https://www.casio.co.uk
https://www.citizenwatch.co.uk
Finally, I'd say to check out Christopher Ward, they are a British watch company and make some great pieces (although possibly out of budget), although If you sign-up to the free Loupe magazine you'll get sent a £100-£150 voucher to use on their website (the magazine is actually pretty nice too).
Christopher
Ward:- https://www.christopherward.com
As far as shopping goes you are pretty safe with a lot of the recognised high-street jewellers, Goldsmiths, Beaverbrooks etc but if you are unsure ask on here and people should be able to say if it's a safe site or not.
Hopefully that's some food for thought and doesn't overwhelm you haha, once you've settled on a few models feel free to pop back and ask any questions you might have around a specific brand, movement etc and everyone here will be happy to help. The best advice I can give though is take your time and try not to impulse buy, back when I started I fell into that trap and bought watches that never even saw my wrist.
Good luck and remember it's a slippery slope, I wouldn't be surprised to hear that your £400 budget grows to £500-600 after you've shopped around a little to see what's out there, one thing that I've found useful to to watch a lot of YouTube videos around specific models I'm interested in to get a little more info before taking the plunge and making a purchase.