If you want to use an existing SIM then the phone will need to of SAME network provider - or be an unlocked phone. Best new unlocked phone is the Motorola G (either cheaper 3G at about £130 or the slightly bigger 4G @about £150). Bril phones.
If you go for a phone from a network provider same as your current SIM it will be cheaper because they will almost certainly have simpler phones. I am not sure whether you can then simply use your old SIM. A new phone from them will have new number when purchased but easy to migrate your old number over. The phone shop will usually do that for you.
You do not need to worry about your present SIM being the larger type. Either the phone shop will cut that to the small size (they have a cutter to do that and did that for me when I bought the Motorola) or of course if you are migrating the number from your old SIM to a new one the size is not an issue.
5 Precepts of Buddhism seem appropriate. Refrain from taking life. Refrain from taking that which is not given. Refrain from misconduct. Refrain from lying. Refrain from intoxicants which lead to loss of mindfulness