Unlike UK which is totally secure by NATO alliance with the US and surrounded by mostly friendly European countries, India lies in an extremely unpredictable neighbourhood, surrounded by unstable countries that are hostile to its very existence and their instability can spill over to India.
China, Bangladesh, Tibet, Xinjiang, Pakistan, Pakistan-occupied Kashmir, Iran, Iraq and Middle-East, Afghanistan, Myanmar -none of these regions are peaceful. Pakistan has a flourishing industry where they print fake Indian currency and smuggle it over to India. China is inundating rural Indian markets with cheap, Made in China produce and laying claim to Indian territory in Arunachal Pradesh which they call South Tibet. China's nuclear missile range covers most of Indian cities. So does Pakistan's. If India didn't have its own nukes, we would be constantly bullied and blackmailed by such "nice" neighbours.
Countries like Bangladesh and Myanmar keep sending infiltrators and drug smugglers -a lot of hashish from Afghanistan also ends up here. The Taliban is trying to engage with Indian Islamic militants who try to destabilize the country from within. China is sponsoring Communist militias in Eastern and Central-East regions of India.
Clearly, India is the one whose spending on defence research/space technology is justified much more than UK. You can afford not to make an additional aircraft carrier -India can't. Throughout its recent history, India has never been hostile to any Western government.
Besides, India defence expenditure is only 2~3% of its GDP. Wouldn't you spend that much money out of your household income to keep your home safe from burglars?
Poverty in India is a problem because of endemic factors like rich-poor divide, lack of economic opportunities in many states and a caste system which is more of a social evil. No matter how much money is poured into India, poverty will never disappear. It's like a bottomless pit.
If UK is spending £300 million in aid to India per annum, I would say "Thank You" but it's still not enough -just a drop in the ocean. If some charitable philanthropists in the UK are doing their part in keeping some poor Indians alive -who are we to stop them.