So if you have some experience at buying bitcoins, i think that you have a 90% of your thread resolved in here, i dont know why you are actually asking for help in here.
I Hold BTC and have done for some time.
Honestly, dash is crap, it is only super over priced and i would never put a single dollar in there, it is very risky to invest. This coin was less than $7 365 days ago (a year)
I would like to get some advice on investing into other coins especially Ether and Dash using GBP.
300 GBP is not much money, but we all have to start at some point right? You can not buy much coins and diversify a lot of because that money would be eaten by fees if you diversify it.
My maximum initial investment is £300.00, but may consider splitting this across more than one coin.
Please advise where it is best/good to trade or if indeed the marketplace may be suitable here for some coins.
Why dont you buy as many ether with that money? Just like you said, you want to buy ether, and if you diversify you will end spending more than $1 per transaction, and if you buy bitcoin, well, you are going to need to pay $40 for the fees.