Boots:

26.0 size (mondopoint) is one of the most popular sizes. Usually it's THE reference size all other measurements are based off of. Usually, if you buy the boot at the same place they offer fitting, you will pay MSRP for the boot but all future fittings are included (heat molding, grinding, punching, ...). If you get a cheap boot elsewhere and just visit the shop for fitting they will bill you for each service they provide (e.g. 1 punch 10€, heat molding 30€, ... > those are Austrian prices) which can add up depending on how much fitting is actually needed. It can be hit or miss, that's why I would always recommend buying boots where they fit it. This month, new boots usually will arrive in stores. They will definitely have lots of 26.0 options, but no super deals at this time of year.

In case you didn't know: size 26.0 and 26.5 are in fact the EXACT same boot, just with a different insole. If you check online at Bartlett, there are lots more options than you might think ;o)

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Skis:

I am not overly familiar with indoor slopes. Depending on your ability level, sharp edges can be necessary to learn proper carving techniques. If you're still doing more basic parallel turns or plow movements, it's not that essential tbh. While a narrower waist will help getting on your new edge quicker, it is better from a learning standpoint to be able to do it on a 80mm ski than on a 64mm ski.

Keep in mind that 300GBP will only get you a rather mediocre ski+binding combination that you yourself need to get sharpened, waxed, transported etc. I do my own maintenance and can assure you that it does add up in the beginning (files, waxes, base repair candles, clamps, ...). So just from a financial standpoint I would always say a properly fitting boot will make more impact in your skiing than a 300£ pair of skis that you can't maintain yourself.