htc phones?

According to GSM Arena your phone has:

Card slot microSD, up to 32 GB (dedicated slot)
Internal 16 GB, 1/2 GB RAM

You can't add internal memory to a phone.

You can add a memory card (SDCard) to many phones. Yours will support up to 32GB micro SDCard. You may be able to get the phone to install some apps to the SD card. Google is your friend for advice and learning.

In all honesty, it may be better just to get a more modern phone. There are plenty of offers around or consider buying a Chinese brand phone from the likes of DX.com, banggood.com or everbuying.net, aliexpress.com etc. Google is your friend for advice and learning.

BUT! Do your research first and make sure you get a model that is suitable for the frequency bands in use in the UK. Google is your friend for advice and learning.

On the other hand, if you are feeling brave, you could get a Galaxy S3 i9300, download and install LineageOS 14.1 (Plus TWRP and a basic GApps package) and enjoy a very cheap and quite good phone, on the latest Android Nougat and without the Samsung bloatware. Guess what? Google is your friend for advice and learning.

Acp
August 2017