Some great suggestions which some have made, but I'll second are:
Planeswalkers decks. Costing so where around £12 -14 depending on where you get them. It's a ready to play deck with a shiny promotional card of a key in game character. It also has 1 or 2 booster packs inside to help them customise the deck.
A deckbuilders toolkit costs more around the 15-18 mark. It's not a ready to play deck. Instead it's a kit containing the basic elements of deckbuilding, as well as booster packs of current sets.
There's a bundle pack sometimes listed by its old name of fat pack. This is better for more established players. It has 10 booster packs as well as counters and funky extras. Expect to pay around 35 for that.
Lastly there's the holiday box. It's a Xmas release of the most recent "throne of Eldraine" set. It's a premium version of the bundle above. Slightly more expensive, but it contains an extra collectors booster that has special prints of current cards.
I see several other enthusiasts have also commented, and you might be feeling a bit overloaded with information. But here are some key things to note:
New players are best of starting out with recent sets. There are very many types of play format, but the easiest to find other players for is "standard" which is made up of the newest and easiest to buy sets.
Accessories are as welcome as the cards themselves. A nice playmat to stop your cards slipping around in-play, or some good sleeves to protect cards from shuffle damage are good purchases.
The most recent sets are
Throne of Eldraine
War of the spark
Magic 2020 core set
If unsure stick to these
There are some really good online retailers who specialise in Magic (it really is that big). I can recommend Chaos Cards, and Magic Madhouse.
Magic has a healthy tournament and fan scene and is a great way to make friends. I hope they have fun whatever you get them