Your question has more to do with macro-economics than 'general laptop discussions'. But let's see what we can do.
1. You need to find a product that is sold in both countries - and the products should, ideally, be identical. The best I could find was this ASUS Flip sold by Bhinneka for Rp18,500,000 (NOTE: This is only meant as an example):
https
/www.bhinneka.com/products/sku12416525/asus_zenbook_flip_ux360uak-dq239t_-_grey.aspx
That's £253 in straight currency conversion. On Amazon UK you can get the same ASUS Flip (broadly) for £1,050:
https
/www.amazon.co.uk/ASUS-Touchscreen-Notebook-i7-7500U-Processor/dp/B01N21B5TT/ref=sr_1_fkmr0_2?ie=UTF8&qid=1524386010&sr=8-2-fkmr0&keywords=ASUS+Zenbook+Flip+UX360UAK-DQ239T
2. This suggests that prices are 4 times higher in the UK compared with the Philippines. In real terms, this means that it's cheaper for a Brit to buy this laptop in the Philippines, and more expensive for a Philippino to buy the same laptop in the UK. This phenomenon is called the Purchasing Power Parity. But the PPP also takes into account wages and salaries. So, while goods and services may be 4 times more expensive in the UK, salaries will also be 4 times higher than those in the Philippines.
3. This means that the 'smart' thing to do is to earn your money in the UK and spend it in the Philippines. Tell your sister to hold out for her first pay check and then you can buy the laptop in the Philippines for her and ship it to the UK - even with expedited shipping charges (2 - 3 days) it's going to be a much cheaper solution.
4. As for warranties, a lot of tech manufacturers offer global warranties, which would be perfect in this situation. But even with regional/national warranties a computer malfunction needn't spell the end of the road - the laptop can still be repaired, you just have to pay for it. The smart thing to do is to buy a sturdy laptop that is also easy to open up and repair yourself.
5. Finally, as for smartphones: Most such devices are still available at very affordable prices pretty much anywhere in the world - even considering PPPs (unless your sister absolutely needs the latest Apple device). Other than that, the biggest concern in this case will probably be the phone bill.
Best of luck to both of you,
GreyCatz.