Honestly you shouldn’t need to break the bank at all to be better than the built in speakers in most cases. I don’t want to say definitively because I’m sure there’s some basically scam products out there, but pretty much any sound bar will be an upgrade in 99% of cases.

A quick search shows a number of well known brands with offerings under £100, and one from a company I don’t recognise that apparently seems to have a number of positive reviews and available for £40

You’ll obviously get a further improvement if you go above the budget level, but that’s definitely not to say you can’t get a decent improvement over not having one at all.