Hiking Wear Advice

You could wear leggings under loose pants for warmth.

You can get a pair of waterproof sweatpants made with synthetic material, or buy synthetic track pants and waterproof them yourself with silicone spray sold at sporting goods shops or online.

Bring a knit ski cap—most body heat loss is out the top of your head.

Bring shoes that have good ankle support. You can also waterproof the tops of your shoes with silicone spray.

Bring thermal long underwear for both under your shirt and under your pants. The thermal ski underwear in the ad you attached would be good. A merino wool sweater and a waterproof windbreaker with a hood will go a long way to keeping you warm and dry.

Finally, unless you want to keep your hands tucked in your pockets the entire time you’re outdoors, bring a pair of gloves.

If you want to enjoy yourself hiking in Scotland during April— you need to be prepared for the cold, damp weather you’re likely to experience.

Mountain Warehouse is a UK store with very affordable outdoor stuff. I’m not sure if I’d trust their equipment to last, especially for serious hikers or outdoor pursuits, but I have several pieces of their clothes and they’re great. They have knit base layers you could wear under those pants for extra warmth. Not sure what will be in their stores since the winter stuff is on sale on the website, but it’s worth checking online. I know they have stores in Glasgow and Edinburgh. There are other outdoor stores there as well if you have time to shop.

You may also find that once you start moving, you’ll warm up quickly enough with nothing underneath. I wouldn’t go with jeans because when they get wet, they will stay wet. Maybe just a pair of rain pants over yours will provide enough warmth and are easy to take off when (if) it gets too warm. Mountain warehouse sells those too.

I would recommend getting hiking trousers. Sweat pants and jeans will get absolutely sodden it rains and be so uncomfortable. Mountain Warehouse is good for affordable options as is Decathlon which you’ve linked to above. These type of trousers are also easy to clean mud off with a damp cloth and so you can wear them day after day.

if you’re concerned about being cold some thermal leggings would be good to put underneath. It’s a bit late in the year for thermals to be available in stores so I would buy these online before you travel.

Amazon has several options for rain pants with zippers, (so the pants go over your shoes/boots.) The cost runs around $30. Rain pants do provide extra warmth over street pants. I recently purchased some made by I-Creek.
Have a great trip!

If you’ll be passing through Inverness, Scotland—which is the gateway city to the Highlands—there’s are a number of sporting goods stores in Inverness, that have clearance sale outdoor clothing. If you Google up “Inverness, Scotland Sporting Goods Stores” you can see what they have available.
There are a lot of rainy days in the low 50’s & high 40’s forecast for your dates in Inverness and, having been there, I know the weather is something you need to be prepared for.
Have a great trip!

If you won't be using waterproof hiking boots consider waterproof hiking shoes and gaiters like these: https://www.amazon.com/Hikenture-Waterproof-Lightweight-Dirt-Proof-Breathable/dp/B0CSFPTLC5/ref=pd_lpo_d_sccl_3/130-7989301-5422914?pd_rd_w=TyRkb&content-id=amzn1.sym.4c8c52db-06f8-4e42-8e56-912796f2ea6c&pf_rd_p=4c8c52db-06f8-4e42-8e56-912796f2ea6c&pf_rd_r=GCCAMRR76PJ96NQMD5D9&pd_rd_wg=dlz7V&pd_rd_r=65736b98-2ca4-462c-a459-29ade837fbec&pd_rd_i=B0DLKSSN8W&th=1&psc=1. The gaiters will keep water from seeping into the tops of your shoes when you step into the inevitable bog. They'll also keep your ankles and calves dry.

Jeans are the worse of wear.
If you double em.up with cheap waterproof trousers from Screwfix you will get through.
Aprilis a fickle month,it could snow or be 18c a temp we won't have seen since September! And to us Brits feels Warm indeed

Do NOT wear jeans - they are the worst thing possible . If they get wet they take ages to dry and will also chill you very quickly.

Foot wear.

It looks like you are not going on very long ( i.e. several day long) hikes, but mainly 2 - 3 hours at a time. If that is correct, I think you can get by with a pair of trainers and extra socks, so you can change wet socks. I would strongly recommend trainers you are comfortable in over new boots, you have never walked in before.

Take dry socks in a plastic bag, which can also be used for bringing back your wet socks.

Take some porous tape (e.g. https://www.boots.com/boots-microporous-surgical-tape-125cm-x-5m from Boots) to protect your feet from blisters. Put it on at the first feeling of unease. Before you go, check if it is easy to tear off or if you need to take a knife/pair of scissors.