Well, a British accent doesn’t exist first of all, so that’s my first piece of advice to you.
Other than that, I would highly recommend looking at accent challenges on YouTube to hear a variety of English and Welsh accents. I Google searched ‘tumblr accent challenge welsh’ and a whole bunch of videos popped up. You could do the same for the English part. Hearing them for yourself is the best course of action to take, imo.
Otherwise, links because there is already a lot out there on accents and I don’t think I could make any useful contribution to it all.
- How to Write a Southerner/Slang and Accents
- Irish/Scottish Characters
- English Slang - Regions/Countries
- Butchering Accents
- Writing Dialogue in Accents and Dialect
- FYCD: English Accents
- TWH: Writing a Spanish Accent
- KSW: On Writing Accents
- Writing Helpers: How do you describe accents?
- Little Elle: On Writing Accents
- The Accent Post
- On Writing Accents and Dialects
- On Writing Accents
- Broad Tips on Accent Writing
- Accents in Fantasy Settings
- 10 Tips on Writing Characters with Accents
- Advice: Writing Accents
- Advice: Don’t Write Out Accents
I hope some of this helps…! Best of luck.