In all simplicity:
Yes. There is definitely a difference with those prices.
On the other hand, if you were comparing a £300 and a £400 monitor, then there might not be that big of a difference, or it might even be that the cheaper £300 monitor has a slightly better quality, while the £400 is just overpriced high-tier brand monitor. But £100 is so bottom of the barrel that you simply can't get anything "good" for that. And while the £500 monitor can indeed still be overpriced, it most certainly won't be THAT overpriced.
As for the statement "both have HDMI":
That is irrelevant. Having HDMI or not has practically no impact to the final price, nor the overall image quality. If you personally need HDMI, then that's understandable, but other than that, it can't be used as any sort of indication of quality.
All in all, I would suggest to never spend less than £150 on a monitor. £200-300 is usually the sweet spot. Over £300 is where the enthusiast range starts. These monitors usually have a better combination of a good panel type, screen size, high refresh rate, high resolution, and other miscellaneous features (like adaptive refresh rate).
If you want, you could just tell us the budget, main usage scenario (gaming, office, movies, etc.), any special requirements (like the HDMI port), and the helpful people here will give some opinions on what would be the best choice for you.
If your
previous thread is of any indication, then I would still recommend taking a look at the actual HDTVs.