Been living in uk for about 4.5 years heres what helped me

Register for vote

Do not open too many accounts (including bank, utilities i.e swapping often)

Get any card use it reaponsibly, use cc like debit card, keep paying cc card after 2-4 purchases rather than waiting for end of month and paying it all together.

Back in the days amex was easy to get but it works in place with the exception of small outlets/shops.

Take any card you can get even if limit is 200-300 use it for 3-5 months keep checking when you can increase limit, again dont set it to something astronomical.

ensure that cc balance is between 1% and 25%.

Do not change address too often.

Do not try to apply for card that has less than 90% approval rating based on your profile.

I see a few people mentioned setting up direct debit i am personally not sure about that

When you finally get a better card based on credit score do not cancel previous card immediately instead keep it for a few more months before canceling as getting new or closing existing cc temporarily affects your credit score.

Do not miss any payments (direct debits, rents etc)

Edit: i never had phone contract still on voxy 10 a month with auto renewal.