You can use any exchange rate service, but they like it if you use oanda.com because they get trained on that one for T1 and T2 applications.
Since Sterling is taking a caning right now, you would do a little better by picking today's closing rate, 70p to the dollar and use it for all your evidence going back 3 to 6 months. For Naira that would be 35p per 100 Naira They are not going to make a big deal about it as long as you don't go cherry picking through historical rates. The rate is not as important for visitor applications as it is for PBS and settlement applications.
For representation, you can use the remarks section and add a statement like...
A Sterling conversion rate of xxxx has been used. This was the closing
rate on xx/xx/xx per the internet Oanda service.