Right this second it reports that transferring 10,000 GBP would give you 12,000.71 EUR vs the 11,949.73 EUR you would get by using Wise, which is probably the best known competitor. Seems pretty good to me!
It's worth noting that the rates you see on the pages often have a bit of delay - I believe both Wise and Atlantic would end up having very similar underlying FX rates, since there's no markup on either.
However, with Wise, the more you send, the more you pay in fees - so there's a break-even where it starts to become more economical to use Atlantic.
Seems to me that the biggest risk to Wise and Atlantic is growing so large that the incumbents will change their pricing.
When trying to enter an established market to compete on price as your primary differentiator it's often better to keep your margins higher to start with than offer the steepest cut you think you can afford, and then drop your prices slowly as you build up volume and a war chest.