My iPhone keeps turning on and off like it turns on and off. Can someone tell me?

Okay tbms it turns on and off on and off and then sometimes it shows a red screen for a few seconds then off again and dies out. It's not the battery because I got that changed thinking that it was the cause and it wasn't. So is there anything someone can help me with?

Added (1). The repair guy tried restoring it through iTunes he said it didn't work either it just won't come on it doesn't go pass the Apple logo before shutting off and on again and dying out

IPhone 7 screen replacement is making everything look grey - is this normal?

I had my smashed iPhone 7 screen replaced last week at a local phone fixing store for £70, and I noticed straight away that everything looks desaturated. At first I questioned if it was just my eyesight but upon closer inspection I've realised the actual screen is more grey than the original white home button. It was a lot of money to have it fixed in the first place and I'm not sure if the man who replaced it would do anything about it if I go back in and tell him I'm not happy - as surely this would mean putting on a brand new screen for free? Is it just a super cheap screen he's replaced it with?

I was just wondering if any tech people out there could give me some advice as I'm a student and can't afford to splash out the cash on a brand new screen just for the sake of being able to edit my photography properly.

IPhone 6 won't update to the iOS 11?

My phone is so slow and it won't let me update! It says "you can manage storage in settings" but I only have the YouTube, Instagram, Snap Chat, and Episode apps. I have like 10 contacts and probably 40 pictures in my camera roll?!

Problems charging my iphone?

My iphone won't charge unless i continue to manually hold the charging cord firmly in the phone's charging receptacle. This is my second cord.

Is this website legit?

I've been looking upon websites to buy an IPhone 7 Plus and looking for the cheaper prices and stumbled upon this account.

http://phoneelectronix.com/product/iphone-7-plus-128gb-rose-gold/

The website has no reviews, a weird About Us, and seems sketchy.

$171 for an IPhone 7 Plus is VERY cheap along with it being 128GB is fishy. Never heard of this website but would like feedback as to if I should buy from this website.

I will be bringing a brand new Iphone 8 into the UK when I'm on holiday there, will I have to pay duty for it? - 1

I will be visiting the UK for 3 weeks soon and I will be taking the brand new Iphone 8 that I have now with me, will I be charged duty on it? I'm not British.

Do I have to declare it and if I do, why do I have to declare it?

Added (1). The phone is in the box, un opened, it is a present for my girlfriend in Belgium. Which is where I will be going after the UK. The box is still sealed.