You don’t have any needs, so you really don’t need to spend any money on a smartphone. If your grandkids want you to have one, try suggesting that they give you one. They could club together for a Christmas present, but it doesn’t even need to be new. I expect many if not most of the people you know have at least one Android phone sitting around in a drawer.
Ask around. Most old Android phones are worth little or nothing. You may well know someone who will give you one.
Failing that, do a regional search on a classified website such as Gumtree, or try Preloved or Second-Hand Phones.com etc. (Caveat emptor: I’ve not used either.) You can also buy second-hand Android phones on Amazon.co.uk and eBay, and probably from local “cash converter” shops.
Android is now up to version seven, AKA Nougat, although its eighth iteration, Oreo, is round the corner. However, around a third of Android users are still on version six (Marshmallow), while a fifth are on version 5.1 (Lollipop). Roughly as many people are using version 4.4.4 (KitKat) as are on the latest Nougat. Get the newest version you can, while avoiding any sweets or puddings that appear before those, alphabetically speaking (Jelly Bean, Gingerbread etc).
Before you buy a second-hand phone, check its approximate value.
Carphone Warehouse has a trade-in website where you can type in the product name or IMEI and get a price. For example, it will give you £10 for a fully-working second-generation Moto G, and nothing if it’s not working or otherwise damaged. (Having the IMEI also means you can check the phone’s background.)
The second-generation Moto G is still a decent phone with a 5in screen. It was launched in September 2014 running Android 4.4.4 (KitKat), and it can be upgraded to 6.0 (Marshmallow). Depending on the specification, you should be able to get an unlocked one for £25 to £45.
In any event, avoid smartphones with less than 8GB of storage.
For more advice, see my earlier answer: How should I go about buying a second-hand smartphone?
Cheap new smartphones
Generally, I recommend spending roughly £150 to £250. In your case, I looked for smartphones that cost under £100. The results were mostly Chinese models that I’ve never used or, in many cases, even heard of. It might be worth considering ones with lots of good Amazon reviews, though these phones also seem to have plenty of bad reviews. Examples include the £42.89 Doogee X10 (217 five-star reviews; 136 one-star reviews) and the £46.99 Padgene Stylish (10 five-star reviews; 9 one-star reviews).
The Oukitel C5 Pro might be worth a punt at £69.99 because it handles 4G as well as 3G, and it was selling for £255.99 back in August.
The Vodafone E8 (£49) and N8 (£79) and the Alcatel Pixi 4 (£99.99) are well known in the pay-as-you-go smartphone market, and have been quite well reviewed. They often come with drawbacks. For example, you may be obliged to buy a £10 “bundle” – which you cancel then drop after 30 days – and get them unlocked.
On balance, I reckon your best bet is the 4.5in second-generation Moto E for £89.99. It’s a 4G smartphone running Lollipop. It’s sim-free and unlocked, so there are no hidden charges. If you shop around, you can sometimes get a new one for around £70. (In passing, the first-gen Moto E was reviewed here by Samuel Gibbs.)
Asda’s pay-as-you-go deals were generally very good so there’s probably not much point in changing. However, if you have a standard sim, it probably won’t fit in your new phone. I assume that if you contact Asda Mobile customer services on 2732, they will send you a new multi-sim so you can pop-out whichever smaller size you need: micro or nano. Make sure Asda understands that you want to keep your existing number.
If you want to switch to a different service, you will need to ask Asda for a PAC (porting authorisation code) to transfer your number.