Bike park wales is the go to. One of my mates went there for a whole week,bit of a vet when it comes to MTB and loved every moment. Stick to the blue trails when starting out and you're grand. Good idea to get used to rentals.
And if you ever want your own bike save up as much as you can for a good hardtail with hydraulic disc brakes and a good air fork. MTB is great for the health,helped me get way more fit for milsims )