If you’re investing £50 a month, that’s 6%, notwithstanding that your portfolio may fall as well as rise, so it could compound the losses. At this amount, I wouldn’t use an ISA, but a GIA would seem more appropriate. Additionally if you’re also doing instant orders or buying US stocks with an FX conversion.
If you’re planning on investing higher amounts in the future, or lump sums then I’d go with an ISA.
The important thing is to work out what you can invest, and the cost/benefit of GIA vs ISA as it will vary for each investor.