Not looking a state on holiday.

I agree with much of the advice you've already been given.
Plain well cut elegant clothes. Good
ibfeel put sunglasses, mani pedi, leather accessories only - bags, sandals etc

I'm now in my 50s & over the past few years I've invested in a summer wardrobe. It has lots of plain colours, navy, white, black, khaki, cream. Highlights of gold.

I feel put together now in the the hot weather

How do you get a sun hat to the destination in one piece ?

Braid your hair
Simple clean lines
A big glamorous beach bag nicely hides a big bum. Gold or straw
Straw hat
Woven bracelets

I get my linen dresses from Etsy. They're pricey but I buy one a year and have finished my collection: they do them in loads of pretty colours:

www.etsy.com/uk/listing/966928368/

Neon is a no go, its Primarnis go-to summer palette.

TK maxx often have nice beachwear in. And agree Cos (or H&M if on a tighter budget) do nice neutral cotton and linen pieces

Haven’t bought anything from Boden in years but saw this and thought it looked like a good style and fabric weight

www.boden.co.uk/en-gb/relaxed-tiered-beach-dress-black/sty-d0422-blk

Please do not dye your lovely hair brown. Also fake tan on very pale skin usually looks very fake. You could do a moisturiser with a bit of tan in to take the blue look off though.
For clothes: cotton, linen or silk dresses will always look better than denim shorts. Buy one or two very good pairs of leather sandals and get a beautiful pedi. Then you’ll look fab

Linen. Long/midi linen skirt, loos fitting. Same with a linen dress. Either pastel shades or neutral like GoldenCagedBird suggests. Simple sandals or sneakers, not the trainers you’d use for gym or running. Large-ish and simple style straw hat.

On holiday, I wear loose wafty maxi dresses in cheesecloth or cotton, a huuuuge sun hat and massive shades. Throw it all on in 10 seconds and you still look put together.

have you thought of having your colours done? there is another thread about this at the moment. I'm sure neutrals look lovely on some people - as suggested here - but on me they just look dull and grungy. I look much better in spring colours such as bright blue, pink.

being comfortable is a big bonus. especially if its hot

COS and good accessories.

I had a similar epiphany when I was in my twenties and had severe envy towards some glossy older women in the resort I was staying at.

Here are some rules I’ve tried to follow since:

Neutral, neutral, neutrals. No jazzy flowery cover ups
or summery neon colours. I have a colour palette of beige, camel, black and white for all of my cover ups and summer dresses.

Following on from the above- my ‘evening’ summer dresses are a bit braver, but they are in the autumnal colours that suit my skin tone. Where printed- they are also ‘interesting’ prints if that makes sense. A tie dye,
gold threads, paisley - not the striped, polka dot or ditsy floral that I would have panic grabbed from New Look pre holiday, pre epiphany.

A good linen shirt- try on a few to find a nice cut for you. A couple of linen dresses too. Uniqlo and M&S are good.

Denim shorts that are long enough to show leg, but not short enough to show arse.

A nice pair of metallic sandals.

A nice straw beach bag or slouchy leather bag in a neutral colour.

Great sunglasses- Le Specs are around the £50-70 but look fab.

Not too much make up or overt styling - a good hair oil on your hair, a tinted balm on your lips, a sweep of bronzer with a bit of shimmer.

Sun hat

I think it's about showing less skin but still being cool. So linen trousers and maxi dresses in neutral colours, not too many patterns. Big sun Gad and great glasses. Add some gold Jewelery for the evenings and a nice straw bag. Make sure your nails are done and a nice lip balm

Aspiga maxi sundresses are beautiful wafty cotton and lovely to wear in the heat. You can wear them to the beach, for sightseeing, or dress them up for dinner.

Hi OP, I'm a pale Celt and this question is close to my heart. After years of feeling a state my approach is now to go very classic, almost masculine, in my look, and keep to a simple palette. Cotton, denim, linen, leather bag and a roomy low key bag for towels etc beside the pool.
COS, Arket, and some French brands are where i feel safe. My main tip however is to get a seriously great pair of sunglasses. They really elevate even simple shorts and t shirt.
I am never going to look like the incredible people i see on holidays but at least when i plan ahead like this i feel I've done what i can.
Oh, and a pedi, maybe in a bright red or coral that gives my plainer clothes a little boost.