As you say, the reviews seem to be OK:
https://ww2.feefo.com/en-GB/reviews/streme?displayFeedbackType=BOTH&timeFrame=ALL
Further, their website does state "Our screens come as DIY kits or ready made screens and we offer a fitting service in most of England". That seems to make it clear that fitting is an 'optional extra' (for which could reasonably be expected to pay extra).
However, if you've already agreed to pay an installation charge, suddenly increasing that (to account for time and fuel) would seem to be unreasonable. Even if the principle is accepted, the actual amount (as originally quoted) would seem to be excessive. Even after the reduction, it still appears unreasonable. HMRC would allow 45p x 44 miles for the cost of using the van, which is about £20. Then, assuming that you accept the charge as reasonable anyway, you'd need to add on an hour's labour, at perhaps £15.
The 'missing' £40 also seems to be a bit dodgy.
As I see it though (having just waded through the relevant sections of the Consumer Rights Act 2015), increasing the price that you thought you were going to have to pay would only be unlawful if the company insisted that you were already bound by a contract to purchase their goods and services. If they've simply said "OK, here's the total of what you'll really have to pay if you go ahead - but you can still back out of the deal without penalty" they're probably still on the right side of the law.