No one can know if BTC price will keep raising or not. So, no one can really tell you if it is a good idea to group your monthly £25 purchases into bigger ones. If BTC raises just as much as it has done this year it is not recommended.

I would however consider another solution which you might be on the fence about. What if you get a small loan in the bank for say £300. Transfer those £300 to kraken.com (I assume that you live in the UK and that UK are still part of the European bank transfer system). Make a SEPA transfer to kraken.com and then buy once every month for those £25.

I am not sure what interest rate you are able to get in the bank. Maybe hear your parents about a 3% loan which would still beat the fees in Coinbase etc.

Listen I am not advising you to loan to purchase BTC. I am just advising you to look at the costs related to the purchases you are going to be doing anyways.