Raffle & Lottery has strict rules associated with it and in the case of a Lottery you need a licence which is very hard to get.
Giveaways & Competitions are not as bad. They have less regulation and don't need a licence, and you can profit from them. In the older days it used to be a Premium Rate service where it cost £1 to enter and many TV shows would offer it. But it got into dire straits when a lot of the shady practices in the Premium Rate industry were revealed. As such they lost a lot of popularity and took ages to properly get slowly back to being used, with new regulations.
If you run a competition in this way, you need to have an element of skill. It can be asking a silly question with a fairly easy answer, but it has to have one, otherwise it is a prize draw/raffle/lottery. So you need a question like "Who is the current Formula One World Champion? A - Lewis Hamilton, B - Peppa Pig, C - Doctor Who".
To be honest though, while that is not too difficult to operate and you can get round any major legal issues with ease, the biggest blocking point is finance. Firstly you need to offer either one high value or several medium value prizes to make it worth while. Next, you need to make money from it. And finally, the hardest part is getting enough entries that you turn a profit.
The high value would be a car or holiday so you are talking £7K+ that you will have to purchase or have an agreement with before it goes on offer.
Medium prizes are Iphones, iPads, TV's, Games Consoles, etc and these will require at least 3 or 4 different prizes, and ideally 1-5 of each so that there are multiple winners for each prize.
Making money from it will require a form of payment to enter. If you do Premium Rate entry, you are looking at several hundred a year (at least) in PRS costs, you need to register with the phone regulator (and last time I did it cost £150/year). There are very strict rules and regulations in place as to what and how you advertise the service, how much you can charge and even how the advertising of them. If you fail to do as they ask, even down to using the wrong advert colours, you can face up to a £250,000 fine (and I know of businesses that have received £25K fine for using black/yellow in an advert that made it slightly difficult to read the terms and conditions).
If you try and take credit/debit card payments you are going to have to use a high risk merchant as most payment providers will consider it as a high risk venture and liable for refunds or complaints.
This means going with a payment provider with less favourable terms and may require a £500 up front deposit to cover for any refunds.
Finally, you are going to have to factor in all the costs of the website, advertising campaigns, payment solutions, as well as the prizes themselves, which could mean a £500 iPad might cost you £800 in costs to host it as part of a competition site.
With that in mind, if you have an entry price of £1, you are going to need 800 entries. Now bare in mind that even competitions by the top commercial UK Radio Stations offer FREE ENTRY competitions and only get approximately 3,000 entries, you are going to have to really push hard to hit £1000 in paid entries to cover all your costs and put £200 in profit in your pocket.
If you were to go for it, there are plenty of sites that can help you advertise a competition, from prize blogs to Loquax and Prizefinder, and newsletters showcasing the latest competitions. Competitions are a big community and lots of resources out there for them, but 95% of all the competitions on all these sites are either Free to enter, Purchase Necessary on a famous food brand, or are a paid entry for a famous and reputable TV show like This Morning, etc.
I personally would not enter this market as a business, because it is a minefield and there are so many free options for compers to enter, but hopefully the info you have on this thread will warn you of a few of the roadblocks and problems you need to overcome if you do pursue it.
