You can use this calculator to work it out https://www.gov.uk/maternity-paternity-calculator
18 weeks is early and things are likely going to get harder as you grow. Hate to say it but it will get tougher than this! I would speak to your employer about reasonable adjustments for you. I don't know what you do for work, but you could request more rest breaks, some support with your workload maybe? Not sure whether your job is one that can be done from home? If so, you could ask for that to be factored in. Ultimately, they want you to be at work, so it's in their interest to support you in doing that whilst pregnant. Speak to whoever did your risk assessment and discuss reasonable adjustments. Of course, taking time off is an option, but it might be worth looking at this first if you're worried about how it will affect your mat pay x
Your maternity Pay is based on what you earn in your qualifying period, if your work either doesn’t pay for sick leave or pays SSP then you’ll earn less and therefore qualify for less SMP. If they pay you in full for sick days then you’ll won’t lose anything x