I don't know if it is possible to get a credit limit of £50k on a credit card but it sounds as if it would be difficult. However, it is easy to get additional credit cards once you have one. There are so many offers for cards around at present. I have eight (I think) with a credit limits varying from (from memory) £3800 to £12500, total over £60k. I didn't obtain all these to get a large total, but rather to take advantage of the very low interest rates on balance transfers and you need extra cards because they are only at the low rate for 6 months. Using this method I have paid less than 2 % interest on the cost of my IR. It cost me a lot but I've got the outstanding balance down to less than £3k now! (interest last month less than £5).
However, beware of the pitfalls, you have to read the fine print VERY carefully. I could write a leaflet on how I did it - I learnt several mistakes the hard way!