The number being displayed may look like a UK telephone number (and may or may not be a valid UK telephone number), but that's not the number from which you're being dialled. The most likely source of the call is some scam call centre in India. They typically operate as small call centres, with multiple "agents". An automated dialler setups the call up often simply by dialling numbers at random* from known active ranges of UK landline numbers, and routes the call through different countries to minimise detection and tracing, and also uses flaws in telephone connection protocols to ensure the calling number shown on your phone is whatever they want. Sometimes that's a random UK number (my landline number has been used in this way), sometimes it's the number of a well known bank, ISP, HMRC or other organisation. If the automated dialler results in you picking up a call when there's not a scam agent to conduct the call, then they drop the call - these are silent calls. The others may have a recorded message trying to scare or coerce the gullible into a situation where they can be defrauded, or the call may go straight to an agent to tell you lies that your router is broken, that your DPD parcel cannot be delivered, or that a payment is about to be applied to your Amazon/Ebay/Paypal account.
If the calling number on an incoming call is your aunt Alice, then it's certainly aunt Alice. If it shows any number that isn't a trusted friend or relative, then it could be a scammer, even if that's a number you think is your bank or some other organisation with who you do business. If it's troubling you, buy yourself (for example) a BT4600 handset which can be set to screen all calls that aren't in your phone's memory, and either delay them or divert to answerphone. If somebody from an unknown number really wants to speak to you they can, but 99.99% of scam calls will be effectively filtered.
* A more troubling alternative is if you've been scammed before, then your number is possibly being traded between scammers who reason if you've been conned before, you're a good prospect to target again.
