Preparation/tips for 50 mile ride
50 miles is feasible but it will be hard work.  Think about how tired you felt after cycling 20 miles. Then cycle 20 miles again from that tired start and think about how tired you'll feel then. Then, cycle another ten miles.

The other answers give lots of good advice which I won't repeat. Since you mention trains and coming home every week, I assume that your plan is to cycle home on Saturday and then cycle back to the place where you work on Sunday.

While I think you can do 50 miles in a day, I doubt you can do 50 miles twice in two days. You should seriously consider taking the train one-way, so you're only cycling 50 miles once a week. This also has the advantage that you could travel home on the train on Friday evening and get more time there. The problem is that British one-way train fares are often almost as much as the return (e.g., when I take the train to visit my parents, the return fare is £104 and one-way is £103; no, that's not a typo, seriously, it's only a pound cheaper).

So, I propose this. One week, buy a return train ticket on Friday or Saturday and take your bike home on the train; cycle back to the place where your job is on Sunday. The next week, cycle home on Saturday; take the train back on Sunday. This works because the return half of your ticket is valid for a month. When you feel you can cycle twice in the same weekend, start doing that.