I'm tiling upto the side of the window frame above the cill line and under it, up to the cill. What's the best way to finish the edge of the tiling by the window? I'd rather avoid using trim unless you think it would look bad without. It's a victorian sash window
You say you tiling up to the side of the frame, is the frame proud of the wall? Is there no window reveal?
I prefer mitred external corners (e.g. if your tiling into the reveal) but they can be tricky for DIY. If stopping at the window reveal (if you have one) you can use either tile trim (but cheapo plastic looks crap IMO) or leave a square edge & paint it wall colour but, if you have a window reveal & I’ve understood you correctly, your going to need to plan it out carefully or I don’t see how your going to avoid problems with the bevels at some point.
Can you please advise what tile cutter to get. I'd like to avoid the normal DIY rubbish ones but I don't know which is a good one.
Rubi would be the my choice but the top end ones will be OTT for a “one off” tiling job unless your planning to do a lot more tiling work. A reasonable manual cutter can be had for around £40-£60 & even Rubi do a couple in that price bracket but I wouldn’t advise trying to cut bevel tiles with a manual cutter. A reasonable, budget diamond wet wheel cutter (£60-£100) is a better bet & will be perfectly adequate for what you’re doing.
Lastly, if tiling over the bath, do I start tiling from the bath upwards (leaving 2mm gap for seeling)?
Yes, fill the bath up before applying the silicone leave silicone to cure overnight before emptying the bath. Use only good quality sanitary silicone & the surfaces must be absolutely clean or the silicone won't stick.