Omer Alien has a reputation for fogging. But you can sometimes get black or brown ones for under £20, which makes it worth consideration. Also, the design has changed a little since I bought mine. I think it may just need more cleaning than normal, which I suspect may be the case for your current mask.
I would suggest using a lighter to carefully and thoroughly burn off any residual silicone from the glass. See YouTube for details/examples.
I would then scrub the inside of the lens clean with plain, cheap, abrasive toothpaste, e.g. basic white Colgate.
For Omer Alien, might be worth repeating the 2 steps above. And perhaps again.
I had much less trouble with my Seacsub X-combat. But my current mask, the Technisub Aqualung Micromask, seems popular and highly rated by spearos. If you can get it for £32 or less including shipping (e.g. Amazon), I would recommend getting it. It's a very good mask but in truth there seems very little between spearo masks, the improvements are small, incremental rather than revolutionary not game changing. Also, it's not perfect, I find it a little small for my face but it seals well, doesn't fog excessively and is comfortable enough.
Mask boxes: my two cheapest masks came with the best mask boxes: strong white flexible plastic boxes with good lids that close positively. The Omer came without a plastic mask box

, so price should be less that masks that do. I don't like the glossy, brittle mask box that the Technisub Aqualung Micromask came in, it's harder to hold, close and pack and feels brittle - but it's a lot better than no mask box and has done the job do far.
Cressi Nano seems well regarded. Much like the Technisub I think. £20 sounds like a very good price
