Why was I accepted for a new contract?

Armorica

three aren't a financial services company; they aren't lending people money in quite the same way

A credit rating of 1/5 could arguably be seen as appropriate if you didn't fund your bank account and went on holiday for 2-3 months instead of paying your bills...

Nasqueron

Credit "scores" are total baloney - they are a meaningless number made up by the CRAs in the hope you will give them money for "premium" services to "improve" your rating (with them).

1/5 is as nonsensical as saying you are worth 2 custard creams out of 10 bourbons or that I score you 2.3 unhappy faces out of 11.7 - lenders and people who deal with credit have an internal scoring system and if they consider you worth the risk they will lend to you (if your previous behaviour is long in the past and you have been responsible since).

No-one will get a refund or debt cancelled because they intentionally chose the most expensive contract and highest premium phone available when they have numerous options and explanation of costs before they chose it.