Since there is often fairly high excess charges to pay as well as the ever increasing insurance costs, and generally dogs aren't often 'seriously' ill unless there is an underlying problem or maybe an accident, do you not think it's as well to put away a similar amount to what insurance would cost each month, say £15/£20, into a separate savings account /piggy bank and if you need treatment.
Hopefully there will be sufficient cash available without dipping into any other money when it comes to needing treatment, tablets etc, as long as it's not something major. Putting away £5 each week would be £260 at the end of one year and if possible maybe a few more pounds could be added to top it up to £300.
Or in the case of Wilhemina's friend who pays £58 each month, which seems a lot, especially if an excess has to be paid in the case of a claim.... £50 a month put away is £600 a year and in a bank would pick up a bit of interest in time if claims aren't made for a while!