QVC has a range of lab-grown called Firelight diamonds. They're all certified and you can try it for 60 days before returning if you decide it's not for you.
If you're looking at lab-grown for ethical reasons, you could look instead at antique/estate diamonds. I have a 1.2ct Old Mine (like Taylor Swift's) and a 2ct Old European as well as my original Brilliant cut. If you like big flashes of colour, an antique cut will be right up your street.
Try an independent jeweller for these cuts, but for inspiration, you can check out Jewels by Grace online and Instagram. Goodoldgold.com also has an enormous range, but both are USA-based. GOG has an education section with loads of info on lab-grown and everything else you need to know about buying diamonds. They have a good range of diamond videos on their YouTube page, too.
Pricescope.com has a Show Me The Bling section where people have posted photos of their diamonds (lab-grown, antique and modern). There's a forum with links to good lab-grown sellers, too. If you click through to August Vintage there, you can find a page (search lab) with pricing for a diamond the size and 'C' that you're looking for. The dollar price is in line with the UK pound price.
Don't buy from a High Street jeweller. They just don't sell good-quality diamonds. Hatton Garden has plenty of choice, but beware 'filled' stones. Several jewellers there sell them, but they don't last and will frequently crack and discolour where the fissure has been filled. Also, beware of fluorescent stones. You can't see them under shop lights, but they turn blue under UV light. There's nothing wrong with them and often face 'whiter' than they really are, but they should sell for up to 25% less than non-fluoro. I have both blue and red fluorescent stones. You really can't tell unless you use UV light on them, though if you know what you're looking for, you can see it show up under very strong sunlight.
Don't rush it, either. Every single diamond is different, and you'll know 'the one' when you see her. Good luck with both your search and your life together.